Undeclared means that they have not chosen their major yet.
Often times a student might be undecided about what they want to concentrate on.
Also transfers are sometimes listed as undeclared.
When I transferred initially I was listed as undeclared, even though I was a junior because I had completed my required classes and electives for undergrad. But not all of my classes were counted (though the credits were) toward the new universities requirements. Until those were completed I could not declare my intended major.
Eventually they will have to declare a major. It is not uncommon for people to change their majors. I did as did my brother. We intended one career path and changed minds.
I had a student teacher intern, not actually student teaching yet but into his first semester of the education program, it was introduction so this was the very first education class he was taking. Although he enjoyed his experience and thought I was a great teacher and easy to work with, seeing the amount of paperwork, lesson planning, preparation, managing student behaviors, documentation, grading, and having witness negative conversations with parents (basically parents screaming at me because "not my child") he stated that it was way too much work and aggravation for the pay and he didn't see himself cut out for it despite enjoying teaching. He ended up going into business. He runs a small business that the owner hired him for and he ended up coaching middle school basketball.
My brother thought maybe the medical field was for him, but after a few medical pre-classes (pre-med) he changed his mind and went into accounting.
Home school athletes might have undeclared because they have completed high school requirements but maybe not have been exposed to electives that might interest them into their particular future field.