Main refers to the Main bar on-screen. It is a blue bar with a number next to it. This bar shows how many black charge attacks you can perform. Whenever you use a black charge attack, the main bar will decrease suddenly and the number next to the main bar will decrease by 1 for every projectile released. The main bar will immediately start refilling itself whenever it isn't completely filled. You can still perform black charge attacks whilst it is refilling. Should the number next to the main bar reach 0 (this will happen at the same time as the blue bar reaching 0), the bar will turn gray and you will be unable to perform black charge attacks for some time, until the bar completely refills. You can still perform yellow charge attacks and red charge attacks. When it completely refills, you will regain all of your main and the bar will turn blue again.