What does the slope of a line on a graph of motion refer to?

There are two frequently used graphs of motion: 1) displacement - time and 2) velocity - time.

The slope of a line on a graph of displacement - time refers to velocity.

The slope of a line on a graph of velocity - time refers to acceleration.

The slope is run/rise = Δ f(t) / Δt

So, in the displacement - time graph the slope is change of position / time elapsed, which is the definition of velocity.

While in the velocity - time graph the slope is change of velocity / time elapsed, which is the definition of acceleration.

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