It refers to the subshell or sublevel to which an electron belongs and describes the motion of the electron. The magnitude of the angular momentum of an electron (angular velocity x moment of inertia) is related to  by the expression:
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where =0,1,2,3,........,(n-1).

This means that the values of for the subshells of a shell are govermedby n. When n=1, there is only one subshell with the value . When n=2 there are two subshells with the values =0. 1. when n=3 there are three subshells that have values =0,1,2. Similarly, when n=4.. no. of subshells=4 with values = 0,1,2,3. For every value of . there is a separate notation to indicate subshell. For example.

The first four notations :s,p,d and f are adjectives used earlier to identify spectral lines; sharp , principal, diffuse and fundamental but subsequent notations used for >3 proceed alphabetically : g,h.i.j. ...so on.

Knowing the value of n and finding out , we designate a subshell. For example, subshell for n=1,=0 is called is subshell and others are given in the form of  a table

For atoms other than hydrogen, the energies of subshells within a given shell increases slightly with increasing. For example, for n = 3, has values 0,1 and 2 with subshell notation 3s, 3p and 3d, repeetively. For H atom , the subshell energies are equal.
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But for other atoms, the energy increases with increasing , that is :
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