Ever since our ancestors learned to beat sticks on logs to make rhythms, music has had tempo. Tempo gives music its pace and is in part responsible for the way the music feels. Songs using a fast tempo, such as jungle, samba, or hard house, have a certain sense of urgency not found in music of a slower tempo like jazz, trip-hop, or deep house.
Tempo is measured in beats per minute (bpm). Sometimes you'll set the tempo based on the style of music or mood you're trying to create, while other times you'll choose a bpm that matches the tempo of the samples or drum loops you intend to use. In any case, Logic's metronome clicks along to the bpm you set for your song, and Arrange window Audio Regions snap to a grid with the timing determined by the song's tempo, so setting the tempo is one of the first things you'll do when creating a new song.