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!
Ever look at the horizon when the sun is rising or setting? Know why it's called the horizon? It's a horizontal line! And just like the horizon, horizontal lines go straight left and right. In this tutorial, you'll learn all about horizontal lines including their slope and what the equation of a horizontal line looks like.