Without knowing the specific BB or bike, tightening up is the likely cure. I assume you mean movement laterally, ie from left to right across the BB shell when you kneel next to the bike. In which case check for movement of the cups (either side), being the bearing shells which screw into the frame itself. Movement in the cups can often be better detected by resting a finger on the cup as you wobble the cranks with the other hand. Anyway, either this - or the actual cranks bolted onto the centre spindle - are the most likely culprits, but get it seen to smartish.
Doing nothing will (in the first case) bugger your frame, and (in the second instance) probably bugger your cranks.
Either is a relatively easy diagnosis and fix.
.
