Plain English Meaning
Cross-control means the stick and the rudder pedals are working against each other — you're banking one way while pressing the rudder the other way. The airplane ends up flying out of balance.
Full Definition
A control input condition in which the ailerons and rudder are deflected in opposite directions relative to the turn — for example, left aileron held with right rudder. In a gliding turn, this typically appears when the pilot uses aileron to bank or hold the bank in one direction while applying rudder against the turn, producing uncoordinated flight and an increased risk of a if the airplane is stalled in this condition.