Bhante Pemasiri is an extraordinary teacher who elucidates the meaning of the dhamma in a clear and insightful way, there are a collection of wonderful teachings on the Karunā Sevena website.
Here is an extract of a teaching on metta he gave to his monks, nuns and disciples in Sri Lanka: (for the rest of the teaching go to Mettā.)
Sukhino vā khemino hontu, ~ sabbe sattā bhavantu sukhitattā!
So "sukhino vā" refers to "may all beings be [sabbe sattā bhavantu] well and happy," "khemino" is... refers to the fear, not having any fear, "may all beings be without fear." It is not good to give anyone fear or, a kind of agitation, to anyone. So a person who is a noble person and one who is practicing like this with the qualities required earlier, will not give any kind of fear to anybody else. So "khemino" is that sense of security, there is no fear. Behave in a way as not to cause fear in others. Fear can arise from many causes. Some people are sort of naturally suspicious. That borders on a sort of mental illness where they are so suspicious that you know of anyone who talks to them or even looks at them. That aspect of fear, which... So a person who is walking this path and training in this manner, even to that kind of person, his behavior will not affect even such a person with these kinds of paranoid tendencies. And he will be extra careful not to kind of... Not to give fear to even such a person... who is prone to that kind of fear. There are people in this world who really get lot of joy out of scaring others, putting fear into them. Such a person when they come close... also you know - generate this fear and dread in others. So then such a person when there is a possibility of... There is fear and dread toward this person, sort of a bully type of a person, then there is no mettā in one's mind and heart. In forest you find this with animals. They are... Particularly fond of people don't arouse fear in them by looking at them and so on. You just live in a way that doesn't cause fear to them. Example when children stare at bird's nest. The bird is actually aware of it and is very fearful. So that even a staring at somebody... something like this can make fear arise. That type of behavior also must be avoided. That comes under sukhino and khemino, this security and sense of well-being of others. It's impossible to live in forest and so on without this feeling of security and safety and happiness, so these qualities have to be developed.