inertial reference frame
A reference frame is simply a point of view for measuring where and when things happen — a chosen origin plus a set of rulers and a clock. An inertial reference frame is a special, well-behaved kind: one that is not accelerating. In it, an object left alone, with no net force on it, either stays at rest or keeps moving in a straight line at constant speed — exactly what Newton's first law promises. A lab coasting through space with its engines off, or a train gliding at steady speed on straight track, is inertial.
The test is Newton's first law of inertia: in an inertial frame, free objects do not spontaneously accelerate. A frame that is speeding up, slowing down, or turning is non-inertial, and in it things seem to accelerate for no visible reason. On a braking bus you lurch forward though nothing pushed you; on a spinning merry-go-round you feel flung outward. Those 'phantom' pushes (fictitious or inertial forces) are the tell-tale sign that your frame is accelerating and therefore not inertial.
Inertial frames are the stage on which special relativity is written: the postulates and the formulas for time dilation and length contraction all assume you are comparing two inertial frames. Strictly, no real frame is perfectly inertial — the Earth spins and orbits — but a lab bench is close enough for most experiments, and freely falling frames come closest of all.
A jet at cruising altitude, flying level and steady, is very nearly an inertial frame — drinks sit still on the tray. The instant it hits turbulence or banks into a turn, it becomes non-inertial and everything jostles.
Steady, straight-line motion is inertial; any acceleration or turn breaks it.
Constant velocity is inertial; constant speed on a curve is not. Rounding a bend at a fixed speedometer reading still means accelerating, because the direction of the velocity is changing, so a turning car is not an inertial frame.