Mechita is the diminutive form of mecha, which means 'fuse' or 'wick'. It's an endearing and somewhat unique term in a romantic context.
In the song, Christian Nodal sings, "Tú eres la mechita que enciende mis ganas" (You are the little fuse that ignites my desires). He uses mechita to describe his beloved as the spark that ignites his passion and longing, making it a charming and memorable metaphor for inspiration and desire.
Nodal spends the whole song failing to explain himself. He knows when it started, "Desde el día en que te vi" (since the day I saw you), and he knows where it left him, "De tus besos un esclavo" (a slave to your kisses), but the part in between is simply missing: "Yo no sé cómo pasó" (I do not know how it happened). The chorus keeps circling back to that shrug and keeps landing in the same place anyway, "Pero me siento feliz" (but I feel happy).
The bridge is where he stops shrugging and starts listing. "Tú eres la mechita que enciende mis ganas" (you are the little wick that lights my want) is the good one, and he follows it straight away with "el encendedor que me enciende y me apaga" (the lighter that switches me on and off), which is a lot of fire for two lines. He also calls her "mi luna y mi fin de semana" (my moon and my weekend), a pairing that should not work and does. Only at the very end does he ask for anything back: "Y no busques a nadie, mi amor, que yo soy solo tuyo" (and do not go looking for anyone, my love, because I am only yours).