On the Y axis of a distance time graph is velocity- speed travelled in a give direction. On the X axis is time taken from start. Note that negative velocities mean something is travelling in the opposite direction to that of the positive velocity.
A line going diagonally upwards shows an acceleration, if it is straight it is a constant acceleration. This is because acceleration is change in velocity over time.
A line going diagonally down wards shows a deceleration. A straight downwards line shows constant deceleration. Again deceleration is change velocity over time but the velocity is decreasing.
The steeper the line the more rapid the acceleration because the velocity us changing over less time.