The surdo is the the deep bassy heartbeat of samba. They mark the time for the bateria and dancers.The first samba drum you hear as a bateria approaches from a distance is the surdo. These are the biggest drums of the ensemble.
Most groups use 3 sizes of surdos. The "primera" or first surdo is the biggest with the lowest pitch. The "segunda" or second surdo, is the middle sized with the middle pitch. And the smallest is the "terceira" or third surdo with the highest pitch.
Primeras and segundas mark the time. As most N. Americans think about samba in 4/4, primeras play on beats 2 and 4, and segundas play on beats 1 and 3. Mocidade Independente de Padre Miguel and São Clamente are notable exceptions to this, they have these reversed.
The terceira plays syncopated rhythms over the top with emphasis on beats 2 and 4. These rhythms along with the caixa rhythms can be specific to each samba school. It's pretty cool to be able to tell which bateria is playing just by hearing the caixa pattern and terceira!