Balanced motor?

What does it mean to "balance a motor"?

All reciprocating parts, pistons, connecting rods, being weighed and match balanced, also having the crank speed balanced.
Just to name a few.

it'll make the engine run smoother and therefore more efficient, also an engine running with bad balance will make your steering wheel shake like the front end is falling apart

How about vibrating everything loose also?

oh yes definately!