This is the opening section of the song, which sets the mood and introduces the musical motifs. It often features a horn section and can include elements from the song's main themes.
After the intro the cuerpo begins: the actual song with the verses. Only after that, in the montuno, comes the groove that people dance to most.
Use the intro to listen: what is the tempo, where is the beat, what kind of song is it? Many dancers wait until the groove has settled before they start.