Welcome. The crankarm lives in the bottom bracket. If the BB is too loose, you'll get side to side play, or slop, whatever you want to call it.

How to test: Grab each pedal or crankarm and try pulling sideways. You shouldn't be able to.

If you can, chances are your bottom bracket is too loose.

You'll need to remove the non-drive-side lockring with a lockring tool and crankarm with a crank puller tool to get at the bottom bracket.

There's a fixed cup on the driveside and an adjustable cup on the non-drive-side. When you pull the non-drive-side crankarm off, it allows access to the adjustable cup. To adjust the BB only, you always start with the adjustable cup.

Using a bottom bracket tool if necessary - sometimes an adjustable wrench will work - you can tighten the adjustable cup to the point where there is no play, but the spindle still spins freely - no binding when the pedals turn.

Then reassemble in the same order, making sure there is no play and binding simultaneously. If there is, go back and adjust again.

If you haven't done an overhaul in awhile, now would be a good time as long as everything is apart. Take the ball bearings in each cup out, clean them with solvent (iso alcohol is good), and repack with fresh grease.

Lots of videos around if you prefer visual instructions. You can also ask questions here if you get stuck.