Commonly translated as "loving-kindness", applicable to the post, but even more simply as "friendliness" to yourself and others. It's crazy the feelings that can come when you sit, say, and feel the effect of phrases like "may I/you be happy", "may I/you be at ease". This isn't a game where we try to get points for being nice for an afterlife, but somewhat of a compounding way of looking at life and interactions with others.
There are many quick start posts, but this is a good one [1] to follow along. Rob Burbea has many talks about mettā, and these [2] are a good intro series.
[0] https://en.wikipedia.org/wiki/Maitr%C4%AB