Please tell the meaning of big drop at 20 second and also draw acceleration time and position time graph ?

Dear Student

Here graph represents v-t graph for a body which starts from rest and keep on increasing till 10 s and reaches to 50 m/s. Then till 10 s to 20 s its velocity is constant.
Here big drop at at 20 s in graph represents that once its velocity is 50 m/s at 20 seconds, body changes its direction. It moves in opposite direction.

Acceleration is the slope of the velocity time graph

So From 0 to 10 s, a = (50-0)/10 = 5m/s² .
From 10 s to 20 s
a = (50-50)/10 = 0
Now 
At 20 s body changes its direction and
a = (-50-50)/20 = 5 m/s²
For 20 to 30 we can not calculate a as values of v are not in graph.
So a-t graph  is shown below.

For position time graph we need distance in time
The displacement of this object is the area of the velocity time graph.
From 0 to 10 s
s = 1/2 (50) (10 ) = 250 m
From10 to 20
s = 10 x 50 = 500 m
At 20 s body changes its direction and for 20 to 30 we can not calculate s as values of v are not in graph.

So position time graph is shown below


Regards