Unscripted talking, usually by a broadcaster. From the Latin ad libitum ‘at one's pleasure’.

(adj) with little or no preparation or forethought(v) perform without preparation(n) remark made spontaneously without prior preparation(adj) said or done without having been planned or written in adv [..]

Spoken words (sometimes witty comments) said out loud that are not in the script. They can also be given “off the cuff” when another actor forgets a line.