What does it mean to say that rational beings should be treated as ends and not as means?

One's "ends" are the objectives one is trying to accomplish. "Means" are what we do to get to those objectives. For example, if someone says, "My objective is to get into that house, and I will use any means to that end." he means that he will do anything to get in there.

We talk about "using" or "employing" means to achieve our goals, which means that Words we associate with means are nouns: "I'll use a screwdriver", "I'll use my acting skills", "I'll employ the Vulcan Death Grip", and so on. Where these are concrete objects, like the screwdriver, we don't care what happens to them as long as they help achieve your goal. Many such objects are disposable.

We also sometimes talk about using people in this kind of context: "I'll employ a high-powered lawyer to get me out of jail", "I'll use this rich widow to get the money I need for my business", "I'll use a hostage to keep the police from capturing me." Now using people in this way may not mean they are disposable or even be to their detriment. The lawyer may profit from defending you. The rich widow may profit from her involvement in your business. The hostage may get away with his or her life. But not necessarily. And when you are using people this way, you don't care about what happens to them. It doesn't matter if the lawyer gets cheated out of his fee so long as he is successful in defending you; it doesn't matter if the widow is left penniless so long as your business is funded; it doesn't matter if the hostage dies so long as you get away from the police.

This is what is meant by treating people as means. It means using them like tools and disposing of them if necessary. But if you consider as one of your objectives the well-being of those people, then they become the end. That is what the quotation means, that to a moral agent the well-being of others is a goal in itself, and our dealing with people or other rational beings should be on this basis, rather than using them as tools to achieve other objectives.