If you mean this in the terms of system of equations, then the answer is pretty long!
When you have two equations with two variables that are the same(say x + y = 14 and 6x + 3y = 42) then substitution is one method to figure out the answer to what x and y are. The first step is to isolate one variable of one equation, which is of your choosing. For example, I might want to isolate y in the first equation. To do this, I would subtract x from both sides of equation one to get y = 14 - x. Next, you would want to plug in your new equation to the original form of equation two. So, in 6x + 3y = 42, I would change the equation to be 6x + 3(14 - x) = 42. From there, you would carry on the solve that equation like a normal, one variable equation (distributing the three, subtracting 3x from 6x, etc...)
When you get your answer for x, plug it in to one equation to get the answer for y.
Hope this helped!
