Related: classical mechanics, special relativity
A frame of reference with zero acceleration.
When we measure a car’s speed, the Earth is an inertial reference frame (we pretend it’s not moving).
Velocity measurements depend on the reference frame.
Another example: on a train with constant velocity, a ball thrown up in the air has no horizontal velocity to people on the train, but to outside observers, there is.
Helpful because everything is moving.
See also: non-inertial frame