Centinela translates to "sentinel" or "guard". It's a less common word in everyday Spanish, making it stand out in a song.
In this romantic context, the singer expresses a desire to be a "centinela" on his lover's lips ("Quiero ser en tus labios centinela"). This beautiful metaphor suggests he wants to protect her words, be close to her, and perhaps even be the first to taste her kisses, adding a poetic and devoted layer to his affection.
Alvaro De Luna's "Juramento Eterno De Sal" is a super romantic and a little bit obsessive song about an unforgettable love. He spends his nights awake, writing her name in his head and drawing her body on a napkin, imagining a world where she can get lost. He wants to see her dream under the stars and be the 'sentinel' on her lips.
He's convinced she'll want him back, promising that the desire to dance will return, along with memories of their 'crazy nights traveling through Portugal.' He vows not to let her miss 'the kisses we gave each other, promising not to forget.' He wants to get lost in her mouth one more time, 'melting our bodies like the sun at dawn,' all sealed with an 'eternal oath of salt.' It's clear he believes their connection is powerful and lasting, even if it's a bit one-sided at the moment!