Mastermind (board game) - Five-guess algorithm

Question

The algorithm (from here) -

1. Create a set S of remaining possibilities (at this point there are 1296). The first guess is aabb.

2. Remove all possibilities from S that would not give the same score of colored and white pegs if they were the answer.

3. For each possible guess (not necessarily in S) calculate how many possibilities from S would be eliminated for each possible colored/white score. The score of the guess is the least of such values. Play the guess with the highest score (minimax).

4. Go back to step 2 until you have got it right.

I confused about the 3nd step -

what is mean -

how many possibilities from S would be eliminated for each possible colored/white score

what is the "correct answer" and the "guess" here ?

Can someone clear it some more ?