Whether they (the treads, not the stairs) can be repaired easily depends on the construction. Stairs come in two types: open stringer and closed stringer; the open style has one or both ends of the treads clearly visible and the spindles attach to them directly, a closed style is one where you cannot see the ends of the treads as they are set in grooves in the stringers and the spindles rest on the stringer rather than the tread.

Do you have access to the underside of the stairs, say from a basement running under that one? If you can access the underside, you will be able to tell what the problem is: insufficient support, loose treads, or something else. Treads are often held in the stringer slots by wedges, which can deteriorate or are missing.

Can you give us a picture of the structure? You can inject glue into the split treads for a quick fix, but if the underlying problem is lack of support, they will just split again.