Dabac is right on the money.
Try removing the chain from the chain rings and using your hand to try to shake and induce movement into the crank assembly. You've got to figure out where the movement you're experiencing is originating. Rotation the crank without the chain will also allow a rough assessment of the lubrication/contamination of the bearings if you feel or hear roughness or grit, odd noises, etc.
Check tightness of the chain ring mounting bolts and the crank arm mounting bolts. It they are tight and play is still detected, it's likely the bottom bracket bearings need adjusted to remove excessive clearance or possibly replaced. The crank axle and/or the bearing cups may also be worn.