A straight line on a speed-time graph means the speed

is constant. this means the speed is not changing over time.

how is this different from a distance from a distance-time graph?

I know its really hard to get the difference but you just have to memorize it

Distance and time graph include

constant speed (going horizontally up)

stationary (not moving) (vertical)

acceleration (a curve going like a half smiley face)

returning to the origin (going up then going back down)

speed and velocity graph includes

acceleration (going horizontally upwards)

constant speed (going horizontally consistently)

deceleration (horizontally downward in other words starts from the upper hand y axis and falls)

stationary (at zero diagonally)