Frame of reference
We have learned about velocity, acceleration, and displacement. But all these quantities need a frame of reference from which they are measured. Let’s consider the following figure

Inertial Frame of Reference

Non inertial Frame of Reference
An inertial frame of reference is a frame where Newton’s law holds true. That means if no external force is acting on a body it will stay at rest or remain in uniform motion. Suppose a body is kept on the surface of the earth, for a person on earth it is at rest while for a person on the moon it is in motion so which is my inertial frame here? Actually, the term inertial frame is relative i.e. first we assume a reference frame to be the inertial frame of reference. So a more general definition of an inertial frame would be: Inertial frame is at rest or moves with constant velocity with respect to my assumed inertial reference frame.
Now we can define noninertial frame as a frame which is accelerated with respect to the assumed inertial frame of reference. Newton’s law will not hold true in these frames. So in the above example if I assume earth to be an inertial reference frame the moon becomes a noninertial reference frame as it is in accelerated motion with respect to earth. But if we want to make Newton’s law hold here we need to take some mysterious forces also known as pseudo forces.
Learn more about these mysterious forces, their origin and application through BYJU’s classes.