Wikipedia is pretty reliable in some areas and is a good source for information.
But the reason why Wikipedia should not be relied upon 100% is the fact that Wikipedia articles are written by and edited by just anyone that wants to edit or write an article.
So the information on Wikipedia may not be 100% accurate so you should always check the information over with some other information from textbooks and other online sources.
Wikipedia is a very helpful website and can partially be relied upon but if you need accuracy from the information do not rely 100% on Wikipedia for that information.
Wikipedia is a website that is edited by anyone and information can change on those articles at anytime as long as the articles are not protected.