What is the function of gate signal in 8254 timer?

The  8254 has three independent 16-bit counters, which can be programmed to. Work in any one of possible six modes. Every counter has a clock input gate input and counter output. To operate a counter ,a count value has to be loaded in count register, gate should be tied high and a clock signal should be applied by clock input .the counter counts by decrementing the count value by one in every cycle of clock signal and generates output.