In a P2P context, share is what is meant by uploading and downloading files.
This is, any client uploading a file (or at least letting other people to download it from it, even if no other client is doing so at that very moment), is sharing that file.
The P2P world is based on that: uploading and downloading files continously to make sure they are always running around the internet and so, can be downloaded by any client at any time.
To learn how to share a/some file/s with aMule, take a look at this article and this other one.
You might also want to know what aMule considers to be a shared file.