I will put in with a simple example. There are 3 variables x,y and z.
The problem is to find the best values of x y and z so that their total value is under a threshold t.
We have another criterion which states that the sum of x and y should be equal to z.
So we create a population of x,y and z.
We calculate the fitness for x+y+z <=t; and evaluate with x+y = z to find optimal values of x,y and z.

fitness=x+y+z. if you want to minimize certain variables then use possible sign otherwise use negative sign

i.e if u want x,y,z pairs to be as least as possible then use positive signs in fitness for that particular variable.