How do I get rid of a small box at the bottom when i take a picture in snap chat?

[AC Question]

I was on snap chat, took a picture, and then clicked something. I started to type the caption for my snap chat, but it wasn't the right keyboard. Instead, the first two letters popped up in a black box on the bottom of the screen. The box does not go away.
I don't know what I hit, and I don't know how to get rid of it. So far, it only pops up once I've taken a picture in snap chat.