About the easiest way to remember what Polymorphism is without running into more techinal jargon is to think of it as the following:
Polymorphism is about making things more the same than different.
For a simple analogy, say you're going to produce two kinds of bags, and these bags will be used to hold either apples or banannas. Rather than creating two entirely separate production lines to create the bags for the two different kinds of fruits, it makes a lot more sense to use the same production line to make both kinds of bags (since they shouldn't be very different, if at all).
In Object Oriented Programming, you're making modules (classes) rather than bags, and they're being used to hold and manage slightly different kinds of data.