Endulza literally means "to sweeten". It comes from the Spanish word for sweet, dulce.
In the song, the lyric is "Endulza hasta el agua salada" (She sweetens even the salt water). This is a beautiful and poetic hyperbole used to describe how incredibly charming and captivating a woman is. It suggests her presence alone can magically transform something fundamentally bitter into something sweet.
This song is all about being completely captivated by someone at a party. The singer from the Mexican group Latin Mafia spots a woman who is just magnetic. After just "una copa de vino" (one glass of wine) and a quick chat, he's hooked. He's obsessed with how she moves, saying "Tú sabes bailar / Se te facilita como el caminar" (You know how to dance / It's as easy for you as walking). The whole song builds this scene of a dance that starts slow and gets more and more intense, until he's ready to risk it all, saying "Vamos, vamos, vamos al cielo / Yo no tengo miedo" (Let's go, let's go, let's go to heaven / I'm not afraid).
What's cool is that it's not just him who sees it. Everyone is watching her. She's the kind of person who "Endulza hasta el agua salada" (Sweetens even salt water). But she's also independent and maybe a little hard to get. The singer knows this might just be a fleeting moment, admitting, "Quiero invitarte a bailar / Pero me vas a olvidar" (I want to ask you to dance / But you're going to forget me). He sees her, she sees him, and he's just waiting to find out "dónde termina el cuento" (where the story ends).