First of all, let's talk about vertical shifts, or shifts along the y-axis. These are transformations that move a function up or down the y-axis, without changing anything else about it.
When you shift a function vertically, you get a new function with the same slope but a different y-intercept. In the equation
y = mx + b
which is the equation for a linear function, this is represented by a change in b.
Just for example, take two functions:
Function 1: y = 2x + 1
Function 2: y = 2x + 3
Function 2 will look exactly like Function 1, except it will be shifted 2 units up on the y-axis, so the y-intercept of Function 2 will be at y = 3 instead of y = 1.
For every 1 that you add to b, the function will move up 1 unit on the y axis. For every 1 that you subtract from b, the function will move down 1 unit.