A walkover is when a player advances because their opponent withdraws before the match starts, through injury, illness or a penalty. It is recorded as a win but not as a match played, so it does not count in head to head records or match statistics.
The distinction from a retirement is the timing. If a player pulls out before the match begins it is a walkover and no result is recorded against them; if they start and cannot finish, it is a retirement, and the completed part of the match counts.
Prize money and ranking points still move. The player who advances collects the points and money for reaching the next round, so a walkover in the early rounds of a large tournament can be worth a great deal to a player outside the top hundred.
For the player receiving one it is a mixed result. An unexpected day off helps a body deep in a tournament, but it also means going into the next round without a competitive match behind them, and players regularly say the rust costs them.