The gate charge refers to the fact that the gate behaves like a capacitor. Needing to be charged up to switch on the fet.
The gate charge tells you how much energy is needed to charge up the gate.
This becomes more of an issue the faster you try and switch the mosfet on/off.
You want it to move between on/off and off/on states very fast because time spent in the transition burns up a lot of energy as heat in the fet. Since the gate needs to be charged you need more current to charge it up quickly and reduce the losses.
The reason this is an issue when driving the fet fast is that number of on/of transitions per second is higher.
So more time is spent in this transition state and more heat lost. This is why mosfets get hotter when you switch them fast.
If you were turning it on/off once per second it would have an entire second to cool down from this transition change heat. if you were turning it on/off at 1mhz it would melt into a ball of fire
Unless you pump 10Amps into the gate to make it turn on ludicrous fast and reduce this heat loss. Note that i'm grossly over simplifying this trying to make it easy to understand. There are limits to have fast you can do things. (The datasheet provides the fastest on/off times possible under Turn on and Turn off times.)