Mettā pāramī : loving-kindness
The Pali word metta is a multi-significant term meaning loving-kindness, friendliness, goodwill, benevolence, fellowship, amity, concord, inoffensiveness and non-violence. The Pali commentators define metta as the strong wish for the welfare and happiness of others (parahita-parasukha-kamana). Essentially metta is an altruistic attitude of love and friendliness as distinguished from mere amiability based on self-interest. Through metta one refuses to be offensive and renounces bitterness, resentment and animosity of every kind, developing instead a mind of friendliness, accommodativeness and benevolence which seeks the well-being and happiness of others. True metta is devoid of self-interest. It evokes within a warm-hearted feeling of fellowship, sympathy and love, which grows boundless with practice and overcomes all social, religious, racial, political and economic barriers. Metta is indeed a universal, unselfish and all-embracing love.
-from (http://www.accesstoinsight.org/lib/authors/buddharakkhita/wheel365.html) by Acharya Buddharakkhita
This is the best summary I could find of a parami (perfection) that is hard to grasp because it contains both the not-doing of certain things and the free flow of other things. There is no direct English equivalent. Within the concept of metta is a complete freedom from the grasping part of our nature.
In practice, it may be easier to start by recognizing this quality in others. The Venerable Henepola Gunaratana is a perfect embodiment of metta; he consistently radiates an imperturbable glow of good-will to everyone he meets or sees, whether they are aware of it or not. When you see the Dalai Lama, you are seeing a manifestation of metta. Perhaps you know others who have an instinctive ability to put kindness first. That would also be metta.
The Buddha taught everyone he met according to their needs and abilities. Not everyone is adept at practicing metta. There can be a psychological hurdle to overcome in being asked to explore the “boundless goodwill” aspect of our own hearts. For some of us, it may be hard to locate this feeling in our experience. For others, it’s as natural as rain.
One way to start practicing metta is to think of someone towards whom you feel a very easy form of goodwill; a respected teacher, a parent or other relative, even a pet. What feeling is that? What are its dimensions and qualities? How does it feel physically? How does it make the mind feel? Can you find any momentum, starting from there? Can you spread the feeling to another being? Possibly even yourself?