On a v–t graph, one square is (width in s) × (height in m/s) = meters. Count the squares, multiply by what one square is worth. Δx = v × Δt for a rectangle.
Area above the time axis is positive displacement. Area below is negative. Displacement adds with signs; distance adds sizes.
On an x–t graph, rise is Δx = xf − xi and run is Δt. Uphill is +v, downhill is −v, flat is v = 0.