Plain English Meaning
Pressing the rudder pedal on the side of the wing that is up. If the airplane is banked left, the right wing is up, so 'top rudder' means pressing the right pedal.
Full Definition
Rudder pressure applied toward the higher wing in a slip or banked turn — that is, pressure on the rudder pedal on the same side as the raised wing. In a slipping turn, top rudder opposes the direction of bank and helps control yaw, prevent the nose from dropping into the turn, or steepen the descent without increasing airspeed.
Context Anchor
Seen in stall, spin, and slip discussions, especially when describing how rudder use affects yaw and stall behavior in a bank.