One of the features that was introduced to YouTube during their transition to "YouTube One" is the addition of a channel trailer. This will be shown to those who aren't subscribed to your channel, giving you a chance to showcase what your channel/brand is about, and what these people perusing your page can come to expect, possibly converting them into subscribers and followers of your brand/presence.

While it's possible to simply use one of your previously made videos, possibly your most popular/finest work, it's definitely for the best to upload a proper trailer for your channel. By this, you can let the viewers know exactly what your channel is about, possibly including clips from previous videos. You can give your potential subscribers a proper taste of what to expect as well as add one or two calls to action within the trailer itself for the viewer to subscribe.