The difference between charging, and activating a sigil is; charging is when you are filling the sigil with your power, and activating a sigil is when you actually go through the process of casting it out into the universe, and you allow it to activate in your subconscious, so that it can begin to manifest your will.
When you charge a sigil it can activate immediately, if it does not have a trigger, or is not going to be used actively, and it will work overtime. This would be a type of passive sigil and would be like if you drew a sigil, then charged it, and then left it somewhere. That sigil would technically be activated, and would be working unless told not to work, or if it has a form of trigger. But, if you drew a sigil on a piece of paper, charged it, and then burned it. It would send out all it’s power all at once in a more active activation.
So to recap:
Charging is when you are filling the sigil with your power. This will give it the power it needs to eventually work.
Activating a sigil is when you actually go through the process of casting out that power. This will allow it to work through the use of your power you built up.
If you simply charged a sigil with power, it would start working unless specified not to, but you do usually have to do something to cast it out if it’s going to be an active activation, and use all of its power all at once.