Spin is
intrinsic angular momentum, the quantum-mechanical analog to the classical angular momentum of an object spinning around its own axis, like a spinning top or the Earth's daily rotation. Each elementary particle has a fixed amount (magnitude) of spin, which is a fundamental property of that particle, like its mass and electric charge.
There are only certain allowable values for the magnitude of spin, namely ##S = \sqrt {s(s+1)} \hbar## where ##s## can have either positive integer or half-integer values, or zero. Electrons have ##s = 1/2## so ##S = \sqrt{3/4} \hbar##. (beware the distinction between lower-case ##s## and upper-case ##S##)
Analogous to the way that we can orient a spinning top so its axis of rotation points in different directions, we can (loosely speaking) think of a particle's spin as being oriented in different directions. We describe this using the
component of spin along a given
reference direction which we customarily call the
z-direction although it can actually be any direction we like. The
z-component of spin is restricted to a set of values which depend on the magnitude of spin: ##S_z = m_s \hbar## where ##m_s## can have values from ##-s## to ##+s## in steps of 1. Electrons have ##s = 1/2## so they must have either ##m_s = -1/2## (##S_z = -\hbar/2##) or ##m_s = +1/2## (##S_z = +\hbar/2##). We call these two states "spin down" and "spin up".
Unlike a particle's spin magnitude ##S## which is fixed, we can change ##S_z## ("flip the spin") by various methods.
For further information see e.g. Wikipedia:
https://en.wikipedia.org/wiki/Spin_(physics)