pre_answer establishes media (early media) but does not answer.
pre_answer is equivalent to a SIP status code 183 with SDP. (This is the same as cmd Progress in Asterisk.)
It establishes media (early media) but does not answer. You can use this for example to send an in-band error message to the caller before disconnecting them (pre_answer, playback, reject with a cause code of xxxx).
That being said, most apps that require media (like playback) should do a pre_answer for you. If you are having an issue with a piece of equipment taking a long time to setup a media path, you could do pre_answer, then sleep in the dial plan.