y = mx + b is what's called the slope formula.
It's used for when you want to know what the formula for a line is.
y is just the y value.
Sometimes, a question will ask for you to find a line with some slope and going through some point. In this case, you would replace y with the y value of that point.
However, when you're finished with your equation, y should always be a variable.
m is the slope.
To find the slope, you have to use the point-slope formula. When given two points, you put them into this equation:
When you're finished with your equation, m should be a number.
x is just the x value.
Sometimes, a question will ask for you to find a line with some slope and going through some point. In this case, you would replace x with the x value of that point.
However, when you're finished with your equation, x should always be a variable.
b is the y-intercept, or when x = 0.
This is usually what you'll need to find in y = mx + b. What will likely happen is you'll given everything but b. All you need to do then is plug in the values you're given into this equation.
When you're finished with your equation, b should be a number.
If you have any questions, let me know! I'm happy to edit my response with more information if need be.