what does the grade xb mean in college?

Answer 1

X usually means you dropped the class or were withdrawn. I've never seen or heard of "xb," but I would guess that it means you had a B at the time you withdrew.

Answer 2

Each school has their own grading system. This could mean a withdrawl, a dropped class, or an incomplete. Go to your school web site, search for "student handbook," and find the section on grades.