When the score gets to 40-40, it is known as 'deuce'.
Once at deuce, a player must win two consecutive points to take the game.
The word comes from the French phrase "à deux" - meaning 'at two', as in needing two more points.
If Player A wins the next point the score is 'advantage server'.
If Player B wins the point the score is 'advantage receiver'.
If the player at advantage wins the point, they win the game. If she loses it, the score goes back to deuce.
To shorten matches, players sometimes opt to play 'no-advantage', where the person to win the first point after deuce, wins the game.