Mastermind (board game) - Five-guess algorithm
The text you quoted seems clear as it is.  But I'll try to elaborate on step 3, since you asked:

Let $S$ denote the set of possible secrets (given responses to moves you've made so far).  Given a candidate guess $g$, you run over all possibilities $s \in S$ and calculate the response that you'd get if you guessed $g$ and the secret was $s$ (the number of black pegs and the number of white pegs); this is "the colored/white score".  Now, for each colored/white score that could be received, if you were to get that score, you could eliminate some possibilities from $S$ as incompatible with that colored/white score; the "goodness" of a colored/white score is the number of possibilities eliminated.  The "helpfulness" of a candidate guess $g$ is the minimum of the "goodness" of all the colored/white scores you could possibly get, in response to $g$.  Select the guess $g$ with highest "helpfulness".

In other words, let $R(g,s)$ denote the number of black pegs and number of white pegs you'd get if the secret were $s$ and you guessed $g$ (this is what your quote calls the "colored/white score").  Let $Z(g) = \{R(g,s) : s\in S\}$, so that $Z(g)$ denotes the set of "colored/white scores" you could possibly get if you made guess $g$ (given that the secret $s$ has to be one of the possibilities in $S$).   Now, if you have a "colored/white score" $z$ where $z \in Z(g)$, let

$$G(g,z) = |\{ s \in S : R(g,s) \ne z\}|,$$

so that $G(g,z)$ is the "goodness" of getting a "colored/white score" $z$ in response to guess $g$.  Also, let

$$H(g) = \min \{G(g,z) : z \in Z(g)\},$$

so that $H(g)$ denotes the "helpfulness" of a candidate guess $g$.

Now step 3 says: you should play the guess $g$ that maximizes $H(g)$.