y = mx + b

The equation $y = mx+b$ is called the slope-intercept form of a line. The number $m$ multiplying $x$ is the slope. The number $b$ on its own is the y-intercept, the height where the line crosses the y-axis. Here is an example:

$$y = 2x+1$$

This line crosses the y-axis at (0, 1) and climbs 2 for every step right. You can read both facts from the equation without any calculation.

At $x = 0$, the term $mx$ equals 0, so $y = b$. Increasing x by 1 increases y by m. These two facts explain why m is the slope and b is the y-intercept.