I am late so as said by other's the graph just plot's the velocity of the ball with respect to time.
So what the graph say's. as said it is the plot of velocity vs time so as you should know velocity can be either negative, positive and 0 but speed can't because velocity is a vector quantity and it have magnitude as well as the direction unlike speed which only have the magnitude that's why we have a negative side in the graph also
$Analyzing$ $the$ $graph:-$
You didn't mentioned but i am assuming and also the graph say's that the ball is released from certain height toward's the ground in a free fall.
Now let us assume the velocity toward's the ground to be negative and toward's up to be positive
So at $t=0$ we release the ball and it start's falling and gaining speed and reaches to a speed of $9 m/s$ in $1 sec$ which is due to acceleration $g$. Now it is shown $-9m/s$ in the graph because we assumed and your textbook too that the velocity in downward direction to be negative
Now at $t=1s$ the ball strike's the ground and bounces backward gaining the velocity of $9m/s$ instantly as you also might have experienced while playing with a ball in the ground that when you throw it toward's the ground it bounce's back instantly toward's you. Now you might see a little time gap of few millisecond in the change of velocity from $-9m/s$ to $9m/s$ is because that that time was wasted in the deformation of the ball when it strikes the ground
Now the ball start's rising upward's at $t= 1.5s$ till it reaches to the top and stop's due to the deceleration $-g$ acting on it and reaches to $0$ velocity at $t=2.5s$ and again start's falling from the top and gain speed and same cycle goes again
Hope it helped you