Pre-mining occurs when crypto coins or tokens are generated and distributed before going on public sale. These are usually distributed to founders, team developers, and venture capitalists, or early investors. This is usually to reward those who did the work necessary to develop and promote the coin or token. Pre-mining is also proof that the coin or token is actually functional.
It is the opposite of a fair launch and is sometimes criticized because it can be the pre-cursor to a pump-and-dump scheme.