The tendency of the meaning
to change over time.
The word "nice", for example, is a compliment meaning pleasant, friendly, or sweet; its original meaning, a few hundred years ago, was closer to "stupid". (Webster1913 says it meant "foolish; silly; simple; ignorant; also, weak; effeminate.")
This phrase is used by linguists and philologists to mean that words don't mean the same as they used to.