Spin (ms) is a quantum number that represents a type of angular momentum of a particle, e.g., an electron. The angular momentum that spin represents is intrinsic: in some ways it is analogous to the electron actually rotating, but other factors suggest that is not actually the case. This is part of the mystery of quantum mechanics.
A classification of elementary and small composite particles is based upon whether their spin is fractional (1/2 more than some integer, called a fermion) or integral (called a boson). This division influences the behavior of the particle.
An electron orbiting a nucleus can have either of two different spin values, in addition to orbiting at different levels, this orbiting level called the electron's principle quantum number. A neutral hydrogen atom that is not excited in the sense of having an electron in a higher-than-lowest orbit may still have an electron in either of two spins, +1/2 or -1/2, of which one of the two has a slightly higher energy level than the other. The electron at the higher energy level can flip its spin spontaneously, which produces emission of the 21cm line, a very low-energy transition and photon compared to those commonly-produced by neutral hydrogen.