Cell banks are like carrying an 'emergency battery pack' for your shields. Activating one will give your partly-depleted shields a recharge. It takes a few seconds to work after you hit the button to activate. How fast it happens and how much it recharges depend on the class/rating of the Cell bank you're using. You can't bring offline shields back with a Cell bank, so it's better to hit the button a little early than to hit it a little late.
If you have more than 1 Cell bank module powered ('checkbox' in your Modules power management panel is checked), and you press your keybind for 'Use Shield Cell Bank', your ship will use a cell from ALL the powered Cell banks at once. Normally you would not want this to happen because
1) It's probably overkill, it's likely to get you to 100% shields but you could've saved some of that energy for when you were getting low again.
2) It generates extra heat, more heat for every powered Cell bank you're activating. If you're crammed full of Cell banks and accidentally trigger them all at once, you can give yourself heat damage!
You can carry as many Cell banks as you have Internal module bays for, and only turn on power to 1 at a time. When you use all the cells from that Cell bank, go to your Modules power management panel and turn it off, turn on the next one. Or switch to a full one if you're expecting a tough fight
. 
Take me with you!
