Channel Trailer

You set it in Studio under customisation, and YouTube shows it to visitors who are not subscribed, with a separate slot for a featured video aimed at people who already are. Someone who clicked through from a video is asking what else is here, so the trailer has to answer that quickly. A long introduction covering who you are and how long you have been doing this loses them.

The other common error is leaving one up for years. It ages faster than anything else on the channel, and a trailer describing a format you quietly dropped does more harm than an empty slot.