Used turns
When you perform a turn with a character, it's set to 1 on the editor. Set it back to zero to be able to perform another turn with your character. If you got a maximum of 2 turns, make sure your are using only a single turn so you have the chance to set it to zero on the editor. Else your char is marked as 'not selectable'.
Selectable
like mentioned in used turns your character becomes unselectable when he has no more turns. Set the value to 1 to mark your character as selectable and you can select him again in the same round.
Turns removed
If I remember correctly it displays the turns that were removed due to any reason. Not sure if it has any benefit to use, I cannot remember exactly every option in every trainer for every game I've trained. But if doesn't make a sense for you and changing it has no effect for you, then just ignore it.
I have found that the "Selectable" option does not do anything, since the editor seems to just hold onto the last actually selectable character. Might be that I am just doing it wrong, but in my experience, it is easy enough to just change the "Turns Removed" for each character to -90, which gives effectively unlimited turns. (at least until you end the turn manually, which resets it to 0) Someone on the Enemy Unknown board posted this once for the EU editor, and I have enjoyed its use ever since!