Mathematics for IGCSE & O level - Graphs In Practical Situations (Section 1 - No. 24)
In the context of travel graphs, a non-horizontal line indicates that:
The object is stationary.
The object is traveling at a constant speed.
The object is changing its speed.
The time has stopped.
Forklaring
Any non-horizontal line on a travel graph indicates that the object's distance from the start is changing over time, implying a change in speed, either acceleration or deceleration.