What is an unconditioned behaviour?

An unconditioned behavior is a behavior that hasn't been conditioned to be meant as a response to a given stimulus.

For example, if you put a pigeon in a box, it will simply walk in circles to try and find a way out. This is an unconditioned response, it's just going to do that behavior on it's own. However, if you rewarded the pigeon to turn in a circle when you give it a que (the stimulus), it will condition the pigeon to spin when given that que, thus conditioning the behavior.