In chess, tempo is a concept used to describe which of the two players currently has the initiative, and is therefore able to dictate the pace and nature of play. When a player forces the other player to make a move they did not otherwise plan to make by attacking or threatening an attack, the attacking player is said to "gain a tempo" while the player who was forced to make an unwanted move is said to "lose a tempo." Making a move "with tempo" refers to making a move that forces the other player to respond with an unwanted move. Making a move with tempo thus gains the attacking player the initiative by forcing the other player to waste time.
In some cases, a player can "lose a tempo" due to an unforced error, when they make a bad move and then have to undo the move by retreating the piece they just moved back to its original square.
Because white always moves first in standard chess, white starts the game with a one-tempo advantage, and thus white is considered slightly "better" at the start of the game.