Tumbao is a bass line that is found in Son, Son Montuno, Mambo, Salsa, Latin Jazz and Bomba. Often it is an offbeat rhythm involving two main notes or rhythms. Although there are many variations, in the basic Tumbao the first offbeat is known as the Bombo and the second offbeat is known as the Ponche.
The Tumbao has a special alignment with the very particular clave of the song. The clave rhythm of a song is typically a "2-3 clave" or a "3-2 clave" although there are a few others. The clave is the key to understanding the groove of the song and how to improvise within the song. The Tumbao bass line fits with the particular clave of the song and contributes melodically and rhythmically within the unique pulse of the very specific clave of the song. With much listening and study you will be able to understand more and more about how to play "in clave" with the band utilizing a Tumbao bass line.