Tempo (先手) is the right to keep asking questions. White's three on row 4 is an open three: if White ever gets a free move, it becomes an open four and the game is over. But black has a four of his own — and a four is a question the opponent MUST answer. Playing it first forces white to spend his move on your five-point, and only afterwards does black deal with white's three. The forcing move buys exactly one tempo: enough to survive, and sometimes enough to win.
Solve the remaining tasks to finish this lesson.