Hi Amy!
First, let's clear up the terminology. A "linear equation" in this context means an equation involving two variables, usually x and y, in which both sides only have constants and "linear terms," which, in this case, would be a constant multiple of x or y. For example the equation
-2x+4y=3
is considered linear, since -2x is the product of a constant, -2, with x, and 4y is the product of a constant, 4, with y, and since 3 is a constant. This means all terms on either side of the equation are linear or constant terms, making the equation a linear equation. Other examples of linear equations are
x=4
y=-x-1
4y=x+1
Now, an equation is in slope-intercept form if on one side of the equation is solely the variable y and on the other side the sum of a linear term in x (e.g. 5x) and a constant term. Some examples are
y=2x-1
y=0.5x+6
y=-x+8
The coefficient of the linear term (the constant being multiplied with the variable, e.g. the 5 in 5x) in a linear equation in slope-intercept form is called the slope. So, in y=2x-1, the slope is 2. The y-intercept is the constant term, so in y=2x-1, the y-intercept is -1. In the equation y=2x-1, when you plug the numbers 0, 1, 2, 3, 4 in for x, you obtain -1, 1, 3, 5 for y, respectively. On a graph, these are the points (0, -1), (1, 1), (2, 3), and (3, 5). I encourage you to draw it on a piece of paper. Visually, these points form a line. Since when x increases by 1, y increases by 2, on the graph one can draw the "rise" (how much y increases, 2) and the "run" (how much x increases, 1) so that the slope is the "rise over the run," which is 2/1=2. When you draw the line that goes through these points, that's what it means to graph the linear equation. The y-intercept is the y-value at which the line intersects the y-axis. The point (0, -1) lies on the y-axis and on the line y=2x-1, visually allowing us to see that the y-intercept is -1.
Slope-intercept form makes it easy to graph linear equations. To graph a line, generally you only need two points. The linear equation being in slope-intercept form immediately gives us the y-intercept, so, e.g., if we're graphing y=-2x+3, since 3 is the y-intercept, we can immediately plot (0, 3). To find a second point, we can use the slope. In the case of y=-2x+3, since the slope is -2, when we go to the right on the line by 1 (increasing x by 1), we should be going down by 2 (decreasing y by 2, i.e. increasing y by -2). So, starting from (0,3) we know (0+1, 3-2), which is (1, 1), is on our line. This gives us two points to draw the line through.
Not all linear equations are in slope intercept form. For example, 6=6x+3y isn't in slope-intercept form. If we want to put it in slope-intercept form to make it easier to graph, we need to rearrange the equation so the y is on one side and that the other side only has numbers and the variable x. We can rearrange 6=6x+3y as follows:
6 = 6x+3y
6-6x = 6x+3y-6x (subtracting 6x from both sides)
6-6x = 3y (simplifying right-hand side)
2-2x = y (dividing both sides by 3)
y = -2x+2 (using symmetry of equality)
Now, we can graph y=-2x+2 using slope-intercept form using the same process described above!