Hello Simra Shah,

I would like to tell you that Inertia is the resistance of the object to any change in its motion, including a change in direction. An object will stay still or keep moving at the same speed and in a straight line unless it is acted upon by an external unbalanced force.

For more detailed info about inertia, I would suggest you visit the below wiki thread.

The link for the same is:https://simple.wikipedia.org/wiki/Inertia

Hope this helps and all the very best!