I haven't used them on a keg yet, but the CO2 carts are the same kind you can use to inflate bike tyres quickly. From what I gather, you put the cart in the holder and screw the whole assembly onto the pin valve on the lid of the keg. From what I've read on here, there seems to be some debate about whether or not it's even necessary upon initial kegging, as the minimal headroom and CO2 produced in the secondary fermentation is enough to stop the beer going 'off'. There also seems to be some people who incorrectly assume that they are in fact force-carbing the beer via this means.
Someone correct me if I'm wrong, but the CO2 is only necessary (essential?) as the beer in the keg goes down (because your drinking it!), the amount of oxygen in the headspace of the barrel increases. The CO2 in the beer 'escapes' into that headspace, causing the beer to go flat. Same principle when opening say a bottle of Coke, drinking half and then leaving the other half for a week. By that point it'll be flat. Squirting CO2 into that headspace purges the oxygen and retains the CO2 in the beer, keeping it fresh. Top Tap Kegs also need that pressure to help dispense, but that's arguably another issue.