(continuing from last lesson's notes which ended with "Unlike distance, displacement can decrease, so a displacement-time graph can slope downwards)...

Because of this, the gradient of a displacement-time graph can be negative.
The gradient of a displacement-time graph gives you velocity.
The sign (+ or -) of the displacement and velocity tell you about their direction.
In contrast to this, distance and speed are always positive, and don't tell you anything about direction.