An inertial frame of reference has a constant velocity. That is, it is moving at a constant speed in a straight line, or it is standing still. Understand that when something is standing still, it has a constant velocity. Its velocity is constantly zero meters per second.
A non-inertial frame of reference does not have a constant velocity. It is accelerating.

Pseudo force is also called fictitious force. A fictitious force arises when a frame of reference is accelerating compared to a non-accelerating frame. For example if you consider a person standing at a bus stop watching an accelerating car, he infers that a force is exerted on the car and it is accelerating. Here there is no problem and the pseudoforce concept is not required But, if the person inside the accelerating car is looking at the person standing at the bus stop, he finds that the person is accelerating with respect to the car, though no force is acting on it. Here, the concept of pseudo force is required to convert the non-inertial frame of reference to an equivalent inertial frame of reference.