"Terrible with something" means very bad or not skilled at something. When someone says "I’m terrible with X", they are admitting they do that thing badly, or they have a big weakness in that area. It is informal and common in daily conversation, often used in a modest or apologetic way.
For example:
I’m terrible with names, so please remind me if I forget yours.
She’s terrible with directions and always gets lost.
I'm terrible with faces, I can never remember what people look like.
