Answer:
The slope of the line is zero
The object is at rest
Explanation:
The graph position vs time of an object represents the position of the object measured at different times.
The slope of this graph is given by:

where
is the increment in the y-variable
is the increment in the x-variable
However, we have that:
corresponds to
, the change in position of the object
corresponds to the change in time, 
Therefore, for a position-time graph the slope of the graph corresponds to the velocity of the object:

An object having a flat line on a position-time graph has a line with slope zero: therefore, its velocity is zero, and this means the object is at rest.