A bol is a mnemonic syllable that stands for a particular stroke. On the tabla, Dha, Dhin, Na, Ta, Tin, Ka and Ge each name a specific hand-sound; strung together they spell out a theka or a composition. The phrase "Dha Dhin Dhin Dha" is a string of bols.
Bols are not limited to drums. Sitar and sarod players have their own bols for right-hand strokes (Da, Ra, Diri), and Kathak dancers recite bols for footwork. Reciting bols aloud — padhant — is itself a respected skill.
Because bols can be spoken, a whole rhythmic composition can be taught, memorised and performed as language before it is ever played.