The answer already posted is excellent. All I want to add is a follow-up question you might want to ask, and my answer to it.
Q: OK, so those in particular aren't boosters. But would it ever make sense to use lots of tiny boosters instead of one big one?
A: Yes, for two main reasons.
One is the extreme expense of anything involved in space flight, and ...
Why would these be used instead of just using a larger first stage?
Strapping on differently-sized boosters allows variance in payload mass without redesign of the first stage. The PSLV has flown with no (PSLV-CA), small as shown (PSLV-G), or big (PSLV-XL) solid boosters.
PSLV-CA (no boosters) - can deliver 1100 kg to 622 km sun-synchronous orbit