Could anyone explain what he is doing in this video?

I’m new to Blender and I found a video on Youtube where someone did that same thing that i want to do, but this video has no commentary so I don’t know how to follow along. Could anyone explain to me what the Person in this Video is doing at min 02:00.

Video Link:

Thank You!

looks like he’s adding a mirror modifier. You can find more info here:

He translates vertices with T + X/Y/Z (lock to axis) + numeric value