What is trickle charge?

It's a very low charging current for a rechargeable battery. Usually intended to prevent a battery from ever fully discharging.

Trickle charging - Wikipedia, the free encyclopedia

Basically - just what it says. It is a very, very slow charge that recharges the device. If you are asking in reference to the iPad - you may be able to charge your iPad by using a USB port on your computer - and even though you may see a message that it is not charging - it may be trickle charging anyway - at a very slow rate.

Is it even possible to trickle charge an iDevice battery ? I thought lithium batteries stop charging the moment they are full.