Question

what is y+mx+b

Answer 1

you might mean y=mx+b

that is the slope intercept form of a line.  In this form the variable represent the following

m= slope

b= y-intercept

x = x value of a given point

y= y value of a given point

Answer 2

First, it is y=mx+b

It is the equation of a line.

y= the y coordinate
m= slope of the line
x= the x coordinate
b= a constant if one exists