Metta is a Pali word commonly translated as loving-kindness, goodwill, or benevolent friendliness.
In Buddhist teachings, metta means sincerely wishing for the well-being and happiness of yourself and others—without expecting anything in return.
A simple way to express it is:
“May you be happy. May you be safe. May you be peaceful.”
Metta is broader than romantic love. It is closer to unconditional friendliness and goodwill toward all beings, including people you don't know or even find difficult.
That's also why Metta meditation (Metta Bhavana) involves gradually extending goodwill from yourself → loved ones → neutral people → difficult people → all living beings.