A single hand, as opposed to both hands. Used to describe actions performed with only one hand, or to indicate something that can be held or managed with one hand.
I wish you would stop driving with one hand because it's dangerous.
Usage
Often used with the particle で to describe how an action is performed: 片手で (with one hand). Also used with に to describe what is being held: 片手に傘を持つ (to hold an umbrella in one hand). The expression 片手間 describes doing something casually or as a side activity.