Maitrī is a Sanskrit term (mettā in pāli) meaning benevolence, or love, or more precisely "loving-kindness," the primary meaning being friendship, brotherhood (universal friendship, in Jainism and Patanjali's Raja Yoga, it is the first of the four virtues of these wisdoms).
The translation by benevolence points to the fact that it is an unconditional and non-attached love, which the term love, with all its connotations and meanings, does not necessarily show.
Maitrī in the Buddhist sense of the term does not refer to passionate love, nor to conjugal love, which involve very strong bonds of attachment, whereas the Buddha's loving-kindness implies detachment and a certain serenity free from passions and emotional agitation.
Buddha
It is not that the Buddha despises love relationships between people; on the contrary, we see him giving advice to laymen who address him in these terms: "Blessed are we as laymen who dwell in sensual pleasures, who live amidst the problems of beds and children, who use sandalwood from Kâsi, who wear garlands and use perfumes and ointments, who earn and spend money.
For us who are such people, may the Blessed One teach a doctrine by which we laymen may attain well-being and happiness in this very life and beyond this present life.
Love between two beings or filial love toward one's family is thus quite respectable for the Buddha, but it is considerably limited compared to Maitrī, in that marital love or family love as well as friendship is directed toward a few beings at most, whereas Maitrī, loving-kindness is directed toward the infinity of beings in the universe. Its object is therefore infinitely more vast.
Love linked to passions sometimes arouses negative emotions such as anger or jealousy which do a lot of harm.
Moreover, it ties the individual to a life of family and work as we see in the interpellation of the layman to the Buddha: the layman must feed his family and for that to enter into all aspects of social life and thus turn away from a purely contemplative life. For all these reasons, loving-kindness goes beyond marital love or filial love.
That said, these two forms are not opposed either: one can very well feel love for one's children, parents, spouse, and friends, while cultivating Maitrī, the immeasurable benevolence for all beings.
What's more, in the texts, Maitrī (or Metta in Pale) originates by comparing itself to motherly love. Thus in the Metta Sutta, it is said: "As a mother watches over and protects her only child at the risk of her life, so should every living being be cherished with boundless spirit. "
We start from the love of a mother for her young child and try to extend it to all beings in the universe without partiality. In the idea of Maitrī, it is a matter of going beyond particularisms and limitations and extending this human warmth to all beings.
Maitrī / mettā is one of the four immeasurables.
Maitrī is defined as the benevolent wish that all beings experience happiness and that they also experience the causes of happiness (so that this happiness will endure and spread to all circumstances of life).
In this sense, Maitrī is intrinsically linked to Karuna, compassion, which is defined as the wish that all beings be free from suffering and the causes of suffering. Maitrī and Karuna are somewhat like two sides of the same coin.
The mettā is also one of the ten pāramīs, perfections of virtue; it is the antidote to anger and hatred. It is not listed among the six pāramitās.
The mettā is one of the forty objects that enable, in Theravāda Buddhism, the practice of concentration (see this link for an overview of these forty objects). This practice aims to develop concentration but also a selfless love towards all living beings.
Mettā bhāvanā means development of benevolence. This form of meditation is also used at the end of a meditation retreat to share the merit obtained with other beings (dedication of merit).
