Can someone please explain chi squared and its use in ecology?

With very little detail, it is a statistic. As an example in ecology, you want to know whether your forest grows a typical pattern of mushrooms and toadstools. You read in a book how many there should be of each species and you do a study to find out how many there actually are. You then do a calculation (Observed-Expected)^2/Expected for each mushroom type, add them all up and the result is the chi squared statistic. You then look at a table of chi squared probabilities to determine the probability that your forest differs significantly from the expected.