An inertial frame is a reference frame in which a free particle follows a straight path at constant velocity. This is the setting in which the usual form of Newton's second law applies without additional forces caused by acceleration of the frame itself.
Frames moving at constant relative velocity can both be inertial. In Newtonian mechanics, a Galilean transformation relates their spatial velocities while preserving absolute time; in special relativity, the corresponding relation is a Lorentz transformation.
A freely moving object viewed from an accelerating vehicle can acquire coordinate acceleration even when no new interaction acts on it. Apparent-force terms account for that frame motion. An inertial description is an idealization whose accuracy depends on the spatial and temporal scales of the experiment.
New to topics? Read the docs here!