Topo comes from the verb topar, which means "to run into" or "bump into" someone unexpectedly. It's a very common and informal word used in Mexico, perfect for sounding more like a native speaker.
In the song, after a magical night, the singer hopes to see the girl again, saying "por si es que me la topo" (in case I run into her). This single word perfectly captures his lingering hope for another chance encounter.
Natanael Cano and Junior H's song "Ella" throws us right into a late-night party scene, complete with