Acera is the pavement (BE) or sidewalk (US). The kerb is the edge of the pavement/sidewalk (bordillo de la acera). You might find it in the dictionary as curb, the alternative spelling. Examples: I lost control of the car and hit the kerb. He denied kerb-crawling even though the prostitute recognised him.
pues no creo que se trate del borde de la acera....miren, aquí esta el párrafo completo; es parte de una canción:
i don't know how but suddenly it has been a while
and we're been walking in through taht same front door
undressing like there were no parts to hide
and calling each other names we never had
you never heard
and then an unexpected turn
and we hit the kerb
Here "hit the kerb" is used metaphorically to mean their life went out of control just as your car might if you were driving along happily, taking everything for granted, not concentrating, and suddenly the car went up the edge of the pavement.