Substitution is one out of 4 ways to solve a linear equation (the other ways are graphing, elimination, and calculating). In substitution you are given two linear equations for example:

4x+3y=10

y=x+1

The problem is that there are two variables (x and y) and in order to solve there must only be one variable. So the first step is to chose which equation you want to substitute. In this instance it would be easiest to use the second equation because there is one variable on one side of the equal sign.

Since it says y=… you plug in the equation x+1 for y. After you plug in the equation, you are left with one long equation, and only one variable. Here is the equation after substituting y:

4x+3(x+1)=10

From there you can just distribute and solve like a normal equation. Here are the steps:

4x+3(x+1)=10

4x+3x+3=10

7x+3=10

7x=7

x=1

After you have found the x value (or y value depending on what you choose to substitute) you can plug the value into any equation. In this instance we will plug 1 in the first equation to get:

4(1)+3y=10

4+3y=10

3y=6

y=2

So you then have the coordinates (1,2) which is the point of intersection between to the two lines represented in the original equations.

I hope this cleared things up! I can explain some things further if you are still confused