Answer:
In terms of mathematics, a closed dot on a graph simply represents a set of number that satisfy an equation.
For example, if you were to plot the points (4,5), (6,8) and (7,9) on a graph (This is just an example, I'm not sure this would result in an actual line), upon labeling the graph with said coordinates, the points with a closed dot would confirm the fact that said coordinates/points are applicable to the inequality as well as every point beyond that).
In simpler terms, say your inequality was x≤3 (x is less than or equal to 3), plotting this inequality with a closed dot would suggest and confirm that whatever point you plotted (and every number beyond that) is applicable to the equation. So, you would plot the (closed) point on 3 with an arrow going to the left (because every number beyond 3 is less than or equal to 3.
