What does 'good' mean? I do know, (and so does the questioner!), but I won't try to answer. One person suggested it means beneficial. Even if this is accurate, it's rather useless. Because what does beneficial mean?
Have you ever noticed that a dictionary defines each word in terms of other words? There are not foundational words with God-given meanings that all new words are defined in terms of (at least indirectly). Rather, if you tried to find out what a word meant by looking up each word in it's definition, and looked up each word in each of those definitions, you would only have a longer and longer list of words to look up, and never finish. And not only that, you'd find yourself looking up the same words over and over. You'd have an impossible task involving looking up an infinite number of words.
So three points. Trying to say "good" or any other idea is meaningless or less meaningful because it can't be defined (without an infinite regress) is specious because this applies to all words equally. And even the approach of focusing on definitions of words is simply a bad idea, because you won't get anywhere. But knowledge *is* possible, so an approach that doesn't get anywhere can't be right. And also, no sorts of foundations are needed to have perfectly valid, useful, true, non-arbitrary knowledge.