An enharmonic equivalent doesn't sound like fun, but in fact the term is fancy jargon for a rather simple concept: it is the duplicate of a note, key signature, or interval that is spelled differently. So two notes are enharmonic if they have the same pitch but are named differently. At least, this is the basis for enharmonic equivalents.
To learn how to identify enharmonic changes and invert them on the piano, check out this piano music theory tutorial. We admit that music theory is not fun, but with this easy video you'll learn a vital aspect of piano playing.