On aircraft without aileron differential, when one aileron goes "up" the other aileron goes "down" the same amount.
On aircraft with aileron diff, when one aileron goes "up" the other aileron goes "down" a lesser distance.

If the down-going aileron creates more drag than the up-going aileron, the greater drag of the down going aileron will cause adverse yaw. In extreme cases, the adverse yaw will cause the plane (model or fullsize) to bank one way and actually turn (yaw) the other way.

This adverse yaw causes rolls to look sorta like spirals---to prevent the spiral roll, we adjust aileron movement so that the down-giong aileron moves less than the up-going aileron. Easy to do with a computer radio--if you are using a servo for each aileron and have them plugged in to the right slots. Much less easy if you have to change throws mechanically.

Different planes require different amounts of differential--some require none; usually takes a few flights to get it just right.

Cheers,