Tempo is the speed of a rhythmic line.
Its units are beats per minute, or BPM in short. It tells us how many beats should fit in one minute, and thus how fast they should be played.
In a tempo of 60 bpm, each beat will last for 1 second. If we double the tempo to 120 bpm, each beat will last for 1/2 a second.
Can you tell which of the following is 140 bpm?