Hi:) what does the one highlighted in pink mean?

The acceleration of the car at the instant when t=3s is found by taking the gradient of the tangent at the point t=3.

The tangent are those dotted lines sloping upwards in the graph.

Tangent: A straight line that touches the curve and does not intersect it.

The reason why they wrote instantaneous is because the acceleration (or gradient of graph) is not constant throughout hence it is just at the moment when t=3s.

Gradient is used to find the acceleration of a speed time graph.

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.