https://online-s.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys100/fall10/preflights/1/graphs.gif

For graph (a) and (c), what is the difference when we talk about velocity... such as slowing down or speeding up or moving in positive or negative direction...

The gradient gives a measure of the velocity.

In (a) the gradient is high then decreases. Hence the body is going fast, then decelerates.

In (c), the gradient is low, then increases. Hence, the body is going slowly at first, then accelerates.

Both gradients are positive, hence, it is in the positive direction in both cases.

It is in graphs (b) and (d) that there is motion in the negative direction.