LV or LOVE (Level of Violence) increases HP and AT when it rises, and some levels increase DF as well. This acronym is also misleading; traditionally, LV stands for Level. Sans describes it as "A way of measuring someone's capacity to hurt."
https://undertale.fandom.com/wiki/Stats

I am made o-o-o-o-of
Lo-o-o-o-ove, o-o-o-o-ove
And it's stronger than you
-- Stronger Than You, Steven Universe