"Good" is an unusual word, in that it can mean many things, most words in language have just one, or maybe two meanings, but "good" is different. A "good" blanket is soft and keeps you warm, a "good" fridge keeps things cold.
Early 19th century philosopher G. E. Moore put forward that "good" was an unambiguous non-natural indefinable property. This sounds a lot more complicated than it is. What is meant by this is that to say something is "good" is like saying it is "big" in so far as it is unambiguous (big is not small and good is not bad, there can be no confusion), but unlike "big" it cannot be defined, and is non-natural - that is, a property that cannot be measured. Colour is often compared in the same way, yet although colour can be arguable, ultimately it can be measured (as percentages of purity and saturation).
Of course this is sadly wrong, and trying to compare the quality of "goodness" to "blueness" or "largeness" is ultimately futile.
Someone with a firm grounding in English might recognise that "good" is generally an attributive, not predictive adjective. This is demonstrated clearly on page 39 of the book "Morality" by Bernard Williams.
G. E. Moore also put forward what he called "the naturalistic fallacy" which simply put stated that to describe something as good was to commend it or evaluate it, where as to simply describe its characteristics does not. This idea is closely linked to the idea of "speech-acts", which describes the different actions that speech can perform i.e., to recommend, or criticise, to greet, or exchange information. This "naturalistic fallacy" seems to assume that the act of describing is separate from the act of commending, which does not seem to me to be the case.
To pin down the meaning of the word "good" seems to me to be a futile act, which serves no real purpose, as it simply muddles the meaning and confuses understanding. The meaning of words seem to me to be localised, mobile, and ever changing, it would be like trying to locate and follow a pint of water dropped in a river. Of course approximate meanings are important, as without them language would be a mess, and translation an impossibility but a simple understanding - with or without explanation - is all that is needed, you know what I mean when I say that something is good, we don't have to be able to explain it.
But to conclude, to understand what is meant by "good" is to understand that it acts mainly as a vague evaluation, and as an approximate description of the subject, of which one must have a basic understanding of the criteria of evaluation.