Motion and rest are relative terms☛ So for a person seeing mobile from earth it is at rest.
We know that we need a frame of reference is needed to locate the position of any object. For that we choose rectangular co-ordinate
system
of three mutually perpendicular axis that are $x$, $y$ and $z$ axis. The point of intersection of these three axes is called
Origin 'O'
and is considered as the Reference Point.
The $x$, $y$ and $z$ co-ordinates describe the position of object with respect to the co-ordinate system.
To measure time , we need a clock.
The co-ordinate system along with a clock constitutes a Frame of reference.
So , the frame of reference is a co-ordinate
system , with a clock w.r.t. which, an observer can describe the position, displacement etc. of an object.
Frames of reference are of two types:-