An undefined slope (or an infinitely large slope) is the slope of a vertical line! The x-coordinate never changes no matter what the y-coordinate is! There is no run! In this tutorial, learn about the meaning of undefined slope.
You may be able to guess that vertical lines are lines that go straight up and down, but did you know that all vertical lines have the same slope? In this tutorial, learn all about vertical lines including their slope and what the equation of a vertical line looks like!
You can't learn about linear equations without learning about slope. The slope of a line is the steepness of the line. There are many ways to think about slope. Slope is the rise over the run, the change in 'y' over the change in 'x', or the gradient of a line. Check out this tutorial to learn about slope!