see inertial frame are the stationary frames..

suppose you are obsrving any body flying in air from the ground means it is an inertial frame..

now suppose you are urself in aframe or a vehicle or smthng which is accelerating...then YOU are in a non-inertial frame..

so non inertial frames are the accelerated frames....

 

to apply newton`s laws in a non inertial frame you got to apply pseudo forces