Parallel lines have the same slope.
Mathematically, we say if one line has slope m1 and another line has slope m2 then the lines are parallel if
m1 = m2
You can explore the concept of parallel lines in the following JSXGraph (it's not a fixed image).
Drag either point `A` or point `B` to explore slopes of parallel lines.
You can move the graph up-down, left-right if you hold down the "Shift" key and then drag the graph.
If you get lost, you can always refresh the page.