Question

Can someone clearly explain me what is pseudo force?

Answer 1

Pseudo forces are not real forces. We only use pseudo concept when we deal with objects which are in non-inertial frames.

Answer 2

a pseudo force is also called d'Alembert force, or inertial force or fictitious force.
A pseudo force arises when a frame of reference is accelerating compared to a non-accelerating frame.
it is is postulated to account for apparent deviations from Newton's laws of motion appearing in an accelerated reference system.
Inertial frame :- Frame in which the velocity is constant is called inertial frame.

Non inertial frame :- Frame in which velocity is not constant i.e there is acceleration in the frame,such a frame is know as non inertial frame.


important point regarding Pseudo forces:
1. The direction of Pseudo force is opposite to the direction of acceleration of frame, it has nothing to do with the direction of motion of frame
2. The magnitude of Pseudo force is equal to the body which we are analyzing multiplied by the acceleration of the frame, it has nothing to do with acceleration of the body
3. Point of applicable of Pseudo force is at center of mass of the body which we are analyzing.

some problems based on pseudo force,

Ques.
An accelerator is moving with the acceleration a directed upward and a body of mass m is put inside the accelerator.Find the total normal force acting on the body

Sol:-

Here in the problem we have seen that the accelerator is moving with the acceleration a upward.To solve these kinds of problem we observe the problem as we are sitting on the body,by doing this we can apply the Pseudo force because in our respect frame is no more non inertial. so the free body diagram of the body inside accelerator in our frame is the direction of Pseudo force is always opposite to the direction of acceleration as i mention earlier so 
N=mg+ma is the answer

we can also solve this problem without the using of Pseudo force FBD becomes as shown above and the answer is still same

Ques. A bus is moving with an acceleration of a ,and a pendulum inside it in the rest position when the bus accelerate at a find out the values of angle of elevation of pendulum means Q.

sol : 

suppose you are also inside the bus now we can apply concept of pseudo force and with the help of free body diagram above we got the answer as tanQ=a/g.
The pseudo force ma is comes opposite to the direction of acceleration

** The FBD of problems are given in the attachment files

I hope it helps you ^_^