Note: For Rand's detailed explanation of her ethics, see "The Objectivist Ethics" in Virtue of Selfishness.
Value is the central concept in ethics. Everything else revolves around it. Ethics is a person's means of choosing between actions. Actions, on the other hand, are aimed at accomplishing something. That something is called a value. Rand defined a value as "that which you act to gain and/or keep". It's what you're trying to accomplish when you do something. That can be acquiring some physical good, like a steak or a new car. It can also mean something like pleasure or improved physical fitness. It can be a friendship, or romantic interest. It can be the development of a new skill. And to give an example of keeping something, putting out a fire to protect your house is an example of acting in order to keep it. Anything you act to gain and/or keep is a value, in the widest sense of the term.
That's why values are the central concept in ethics. Ethics is fundamentally about choosing between possible goals/values. Ethics isn't concerned primarily with how to accomplish a particular value, such as how to cook a nice meal. It's concerned with choosing between cooking that nice meal and every other action you could be taking. Ethics is all about choice. And that means choosing between possible values.
We've discussed values before in the section on emotions. In that section, we described how a value-judgment, the evaluation of whether something is good or bad, is the root of an emotional reaction. The value-judgment is formed through a process whereby you evaluate whether something is good or bad. The method of evaluation is something we'll get into later, but the result is what matters as far as the emotions go. That means even if you evaluate something incorrectly, the value-judgment will still lead to future emotional reactions.
When you evaluate something as positive, one kind of emotional response will be desire. I bring this up so that we can properly distinguish desire from value. To desire something is to have an emotional feeling of longing towards it. Valuing something, on the other hand, just means that you've evaluated it as something good, that you would like to have happen to you. It's not emotional. It's cognitive. Of course, some people follow their feelings. So in that case, they would think that just because they desire it, it must be a value. But they're just evaluating it as a positive because of the emotions. There's two separate acts occurring. One is feeling the desire. The second is concluding that it's a value.
Now I want to finish this discussion off by talking about semantics. The word value has a couple of meanings, and it can confuse people if you're not watching the context carefully.
In the widest sense, Rand identifies it as something you can act to gain and/or keep. That's quite broad, and doesn't say anything about the quality of a person's evaluations. It just says that they have evaluated it as something positive, something they would like to gain and/or keep.
Later, when we talk about objective morality, and the ability to judge whether something is actually beneficial to you or not, the word 'value' gets used in a different way. It comes to mean an objective value. We might say "But it really is a value to you". Or "It is of value to you". Both of these talk about a properly evaluated value.
Additionally, some people will use 'value' to mean 'want' or 'desire'. "I value it" can be taken to mean it's something I want. We'll try to avoid this usage because it's not correct. As I already mentioned, to value it is to recognize it as a value, which is cognitive. You may additionally feel desire for it, and that might even be the reason behind your acceptance of it as a value, but it's not the same. "I value it" means "I evaluate it to be positive".