The phrase "feel at home" is an idiom, which is an expressive, figurative statement. When a person says that another person should feel at home, he doesn't usually mean these words literally. Instead, the phrase typically means the person should not feel like a guest or an outsider. Likewise, when a person says he feels at home, this typically means that he feels accepted and as if he belongs in a particular place or with a group of people.
One of the meanings of the phrase "feel at home" is to feel accepted. When a person feels at home, this usually means he feels as if he belongs rather than like a stranger or an outsider. For instance, if an individual joins a group or moves into a dorm building mid-semester, he could feel uncomfortable and as if he doesn't belong. If people are very welcoming to the newcomer, however, and he feels as if he fits in and is one of the already established group, this usually means he feels at home. Generally, people are more likely to remain in a place or group if they feel accepted rather than having consistent feelings of being outsiders.
Often, the phrase "feel at home" means a person feels as if he is at home in a particular place rather than feeling like a guest. For example, if a person is traveling and is welcomed into the home of friends or relatives, he may feel tentative or as if he should be very careful about what he does or says while he is there. Some hosts, however, do not want their guests to feel as if they have to be careful or tentative. To avoid this, they may put a good deal of effort into making the guest as comfortable as he would be in his own home and let him know that he is free to do as he pleases, without seeking any sort of permission or approval.
Usually, when a person wants to make a guest feel at home, he wants him to feel free to do as he pleases, but within reason. This may mean, for example, the guest is free to use the host's phone and eat from his refrigerator, but not eat everything in the refrigerator without regard to others or call so many people that he creates a high long-distance phone bill for the host to pay. Likewise, the host may say that the guest is free to come and go as he pleases, but with the expectation that the guest will be mindful of others sleeping, for instance.