Leave it is, simply, "don't pick up that thing". The most important point to consider is the second piece of the Leave It puzzle, which is to also give attention back to the handler for further information. That Thing can be a dead squirrel, a toy, a shard of glass, or something that you don't want them to take.
Here is trainer Melissa Millett giving a free demo of Leave It. She also shows the end result after a series of progressions with the command.
I recommend working on these skills a few minutes every day, with dogs (and maybe even children....) so when either finds a dead squirrel, this doesn't happen: