Charge refers to the Charge bar on the screen. It consists of a number and 2 bars, a green bar on top and a black bar on the bottom. When you hold the Charge key (Default: C Key), yellow will fill up the black bar, then red will fill up the yellow bar. This indicates which kind of attack can be performed. The green bar will decrease suddenly (and the number next to the charge bar will decrease) and start slowly refilling whenever you use a yellow charge attack. If this reaches 0, it will turn gray and you will be unable to use a yellow or red charge attack until it refills (black charge attacks are unaffected). When the bar is green, as it refills, the number next to your charge bar will also increase, allowing you to perform more yellow charge attacks. When the bar is gray however, it will not increase the number next to your charge bar until it is completely filled, where you will gain all of your charge back.
There are 3 types of attacks that can be used with the Charge bar:
A black charge attack is where you press the ranged attack key/charge key (default is the C key) once, letting go before yellow fills the bar. This is usually the weakest of the ranged attacks. It will consume one Main for each projectile released (the number next to the Main bar will decrease).
A yellow charge attack is where you press the charge key for a short time (about a second), letting yellow completely fill the bar until red starts filling up the bar. Releasing the charge key when red is filling the bar initiates a yellow charge attack. It will consume one Charge for each projectile released (the number next to the Charge bar will decrease).
A red charge attack is where you press the charge key for a long time (about 5 seconds, can be as much as 15 seconds with Hanyuu's red charge attack), letting the bar turn completely red. It will remain red so long as you hold down the charge key, and you can hold down the key indefinitely. Releasing the charge key when red is the only colour will initiate a red charge attack. It will not consume a charge or a main.