Question

what can be explained from the straight horizontal line observe at the beginning and end of the graph​

Answer 1

the quantity is constant .

for eg. if graph is velocity v.s. time graph

and time is on x-axis as

graph is straight horizontal line parallel to x-axis then veloctiy is constant throught the graph i.e. it doesnot changes w.r.t. time.

hope, your concepts are clear now?