A signature is just the text, links, and sometimes an image that you want to appear below every post you make on the forum. Some members have personalized banners. Some show off their player stats. Some have links to useful resources.
You don't need one, but if you choose to set one up just try to keep it clean and subtle enough that it doesn't detract from the threads where you post.
To set your signature, go into "My Settings" by clicking your username at the top. (On mobile I'm not sure where the option is, but you can hit the "full version" at the bottom to switch to desktop mode.)
From there, in the left menu, click "signature". Then add whatever you like to the text box.
It doesn't have a signature option for me☹️