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.
Any particular type of particle is confined to either integer spins (such particles are termed a bosons) or to spins that differ from an integer by 1/2 (termed fermions) and each of these two overall class is corresponds to some particle-behavior characteristics. Composite particles (i.e., hadrons) may act as fermions or as bosons according to the sum of the spins of their constituent elementary particles, e.g., a composite particle made up of two fermions behaves like a boson.
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 principal 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 21-cm line, a very low-energy transition and photon compared to those commonly-produced by neutral hydrogen.