Te metiste is a powerful phrase from the reflexive verb meterse, meaning 'to get into' or 'to put oneself in'. While it can mean physically entering a space, here it's used in a deeply romantic and figurative way.
The singer uses it to describe how someone completely and suddenly entered his life, heart, and thoughts. The line "Te metiste como el agua entre mis manos" (You got in like water through my hands) creates a beautiful image of how effortlessly and completely this person became a part of him.
"Te Metiste" by Ariel Camacho y Los Plebes Del Rancho is an unapologetically sweet regional Mexican love ballad. Right from the start, Camacho wonders how he could ever repay his partner for all the happiness she gives him, asking "¿Cómo te pago todas las noches?" ("How do I pay you back for all the nights?"). He cannot stand the thought of seeing her upset, bluntly admitting that seeing her face ruined by tears ("tu carita destrozada por el llanto") hurts him directly. For English learners, notice the phrase "así como no queriendo" in the chorus; word for word it means "just like not wanting to," but everyday Spanish speakers use it to mean doing something casually or acting as if you didn't even try.
That effortless arrival is the whole point of the track. Camacho claims she slipped into his life "como el agua entre mis manos" ("like water between my hands"), but instead of slipping away, she stuck around forever ("para quedarte siempre"). He finishes with a total surrender, declaring "hoy soy solo para ti" ("today I am only for you"). It is a warm, unpretentious confession of somebody realizing that love completely took over before they even noticed.