I like to say that broadly speaking, Wikipedia is mostly trustworthy when statements are cited, but it's never a source.
There are several parts to this.
Wikipedia is broadly and mostly trustworthy -- Most of the time, especially in articles of broad interest, errors are caught and at least flagged, if perhaps not addressed, quickly. This is what the  notice is about for claims that aren't obviously unreasonable. (Claims that are obviously unreasonable are likely to just be deleted outright. A claim that "US president candidate Hillary Clinton was born on the planet Jupiter" is more likely to be deleted outright than flagged as "citation needed", for example. Extraordinary claims require extraordinary evidence and all that.)
When statements are cited -- This is an important caveat. Yes, anyone can write anything on Wikipedia. Good edits come with clear in-line citations. A statement that properly cites its source can be judged based on the source. As Monica Cellio pointed out, not all sources are created equal, so you need to exercise due diligence here. Beware of circular source references where, if you follow the line of citations and sources, you end up back where you started. You can reduce the risk of this by checking the Wikipedia page's revision history to see when a specific claim was added or cited to a source, and compare that to the publication or edit history of the cited source. The Internet Archive's Wayback Machine can also be useful here.
But it's never a source -- This is something a lot of people get wrong. You should never cite Wikipedia in any authoritative manner. Wikipedia, just like any other encyclopedia, is a summarization, and very often a simplification. In fact, Wikipedia prohibits adding material that cannot be verified using external references (they refer to this as verifiability, not truth). If it matters, then go to the source or to a specialist publication, preferably a peer-reviewed one, and cite that one (or better yet, several).
With the above caveats, Wikipedia is generally a good way to get a general overview of a subject. It's a nice starting place to find out what more you might want to read. If a specific fact is important, you should always verify it against some unrelated work anyway whether you start out on Wikipedia or with a printed encyclopedia. (Real scientists verify the results of others all the time, especially with new results that don't match earlier models.) Do note that this places a larger burden on you than simply checking that the sources listed for the claim on Wikipedia support the claim; you want to independently confirm the claim, not just confirm that the claim can be supported by whatever someone said supports the claim.
Let's say you are writing a report on how airplanes can fly; you might go to Wikipedia, or your favorite search engine, and type "how airplanes fly" into the search box. If you do that on Wikipedia, you might end up on the page about airplanes, and from there you might follow the link to its page about aircraft wings, which in turn will tell you that aerodynamic forces are involved, from where you can follow the link to the page on aerodynamics which goes into some of the gory details, including links to separate pages on subjects such as incompressible flows and transonic flows, along with separate pages on different types of engines (air-breathing pure jet engines, propeller engines including piston and turboprop engines, rocket engines, ...). Those pages, if you take the time to read and understand them, will probably give you a pretty decent idea of the details of how airplanes are able to fly, to the point that you could probably write up a pretty good summary yourself of how it all fits together, which (unless you're well into the upper years of the school system) your teacher would likely be happy with. However, it would not be a good idea to try to design an actual aircraft with just those, as they just aren't detailed enough. (It's unlikely that you'd even be able to get a pilot's license to fly aircraft just by studying those. You'd likely miss out on some details that are important in such a context, and spend too much time on things that are relatively unimportant.)
Which brings me to what you wrote in a comment to one of the answers...
So is it a good website to use when doing research for a project?
Research is a somewhat loaded term. It can be used colloquially as in "learn more about something", but it can also be used in the scientific meaning of "gathering data" or "determining what model best fits the available data". Wikipedia is generally nice for the former, but it's absolutely useless for the latter. When you're doing a school project, it's a gradual change from the former to the latter as you move up the educational system; by the time you're in college or university, you'll be expected to be doing more of the latter than the former. That implies that you won't even be going to Wikipedia's sources, at least by way of Wikipedia; you'll be reading relevant scientific publications directly.
In summary, don't be afraid to refer to Wikipedia, but if it's important for what you're doing, always at the very least check the sources. Consider any statement that doesn't cite its sources to be at best dubious.