# Inertial Frames/Inertial observer

I'm a toddler, please could you explain concepts as simply as possible, no words with more than 10 letters. Thanks.

So I have begun work on Newton's Laws of Motion. I have also been told these laws do not hold unless I am a inertial observer or in a inertial frame of reference. I very vaguely understand that a inertial frame of reference is somewhere that if an object has a constant velocity, it will stay having a constant unless acted upon by a force. This is correct, right?

However, I don't believe I fully understand what a inertial observer is? I would very much appreciate an explanation as to what that means.

Also, I have seen the words internal and external forces crop up quite a lot, I'd appreciate an explanation as to what that is also, thanks again!