Stringers are “the spine” that supports the treads and risers. There are typically two stringers, one on either side of the stairs but based on architecture and design the stringer(s) may also by relocated. The stringer is a physical representation of the slope of the stair.
An open stringer means the treads and risers on a staircase do not terminate against a vertical projection or wall.
A typical closed stringer is a component of the staircase where a tread and riser terminate (the sloped baseboard going up the wall). In a standard application, the approximate dimensions of a closed stringer are 2”x10”.