A spin-stabilized satellite is a satellite which has the motion of one axis held (relatively) fixed by spinning the satellite around that axis, using the gyroscopic effect.
The attitude of a satellite or any rigid body is its orientation in space. If such a body initially has a fixed orientation relative to inertial space, it will start to rotate, because it will always be subject to small torques. The easiest form of attitude stabilization is to give the rigid body an initial spin around an axis of minimum or maximum moment of inertia. The body will then have a stable rotation in inertial space.