Yes it was a bit difficult to explain in 3 words

It's an option to customize the behavior of the popup keyboard when you long press a key, when they are more than 1 alternative character.
If the option is on, you need to keep your finger pressed, and slide it to select the alternative character you want (like HTC keyboard)
If the option is off, you need to release your finger and press again on the alternative character to select it (like Android keyboard)