Setup & Hold Time
When you talk about setup and hold, it means there is a clock edge or similar event that triggers the capture of an input's value. The value of the input must remain stable during a specific window around this event defined by the details of the circuit involved, including relative trace lengths, device sizes, switching time, capacitance, etc.
So you must prepare or "set up" the inputs to the right values before a certain time, and you must then "hold" them at that value at least until a certain later time, else you risk unpredictable / undesired behavior.