What does a Slab mean, when its referred when looking at a houses basement?

No, nothing is wrong. A slab is just a single piece of concrete which becomes the floor of the lowest level of the house. Most ranch house are built on a slab poured onto the ground after it is leveled. In many houses built on a slab, you can see the edge of the concrete if you dig down an inch or two. If the slab is recessed into the ground a few feet instead of being on top of the ground, the portion below grade is considered a basement. Generally, they pour concrete walls onto the recessed slab and seal the concrete against moisture to form the foundation for the 1st floor. If the 1st floor is above grade, it is referred to as a split-level. If the 1st floor is at grade with the slab and foundation walls below grade, the basement is considered a "full" basement. The word slab is just the term for the type of construction which consists of a single pouring of concrete onto which the rest of the house is built.