Reference frames are a concept most often used in special and general relativity. They are essentially coordinate choices on a (spacetime) manifold.
Often they refer to a particular observer, who is chosen to be (momentarily) stationary in the spatial coordinates in the frame considered. This is then called the observer's frame of reference.
A frame in which no forces act on the observer in question, and which is time-independent as well as homogeneous and isotropic in its spatial cooardinates is called an inertial reference frame. Every observer travelling with uniform velocity below the speed of light possesses sich a frame.
If the observer is accelerated and therefore not uniformly moving, it is nevertheless possible to choose a so-called momentarily comoving reference frame, sometimes also called the proper frame.
This tag should be applied to questions that deal with the differing perceptions of observers in special and general relativity, as these arise from the observers using different frames of reference, often leading to situations someone not familiar with relativistic thinking might think to be paradoxical.