The "S" subshell refers to the set of orbitals with an azimuthal quantum number (l) of 0. In other words, it corresponds to the s orbital. The s orbital can hold a maximum of 2 electrons.
The term "degenerate" refers to orbitals that have the same energy. In the case of the S subshell, there is only one orbital (s orbital) available. Therefore, the S subshell has only one degenerate orbital.