Inertial reference frame
Also called Inertial frame
An inertial reference frame is one in which Newton's first law holds, so an observer in it sees an object with no net force keep a constant velocity. Frames at rest and frames moving at constant velocity are inertial; accelerating frames are not.
The AP definition is a test, not a description. AP Physics 1 EK 2.4.A.5, repeated word for word as C: Mechanics EK 2.4.A.5, calls an inertial reference frame one from which an observer would verify Newton's first law of motion. Watch a force-free object: if it moves at constant velocity, the frame is inertial.
You are allowed to assume it. The assumption is written down twice: a boundary statement under AP Physics 1 topic 1.4 says the frame of reference of any problem may be assumed to be inertial unless otherwise stated, and the exam conventions box on the equation sheet repeats it. So in its plain form is safe by default, and questions that break the assumption say so.
Inertial observers disagree about velocity and agree about acceleration. EK 1.4.B.2.ii states that the acceleration of any object is the same as measured from all inertial reference frames. Since acceleration is what forces produce, all inertial observers also agree about the forces, even while they quote different speeds.
In a noninertial frame that agreement fails and things appear to accelerate with nothing pushing them. A passenger slides forward as a bus brakes, and the fault is in the frame. EK 2.6.C.4 goes further with the equivalence principle: an observer in a noninertial frame cannot distinguish between an object's apparent weight and the gravitational force exerted on it by a gravitational field.
What an observer measures in general, and how to convert between frames, is at reference frame.