The magenta point is P1. Its x and y coordinates are on top on the left.
The red point is P2. Its x and y coordinates are underneath.
Drag the red point or the magenta point on the graph and see the display change.
Enter coordinates directly in the text boxes on the left and see the display change when you click the "Plot points" button. Default is to keep slope and intercept to the nearest hundredth when you drag points, so the numbers in the equation don't look so ugly, but if you enter the coordinates directly you can see "all" the digits.
The black point is the y intercept. Its coordinates are at the top of the graph. Click on the graph and then the arrow keys make the black point trace points on the line.