All linear functions can be written as

The coefficient m is the slope of the line, and it is the constant rate of change for y, depending on the change of x.
Consider, for example, the function

If we start with x=0, we have y=3.
Then, if we increase x by 1 step by step, we have

So, as you can see, as we constantly increase x (by 1), y constantly decreases (by 2, which is the slope of the line: -2)