Cobardes means "cowards". It's a very strong word, and in this song, it's used in a surprisingly desperate and unique way.
The singer begs her ex to "Haz las cosas que hacen los cobardes" (Do the things that cowards do). She wants him to stop being kind and instead act cruelly, giving her a reason to finally hate him and move on from their painfully perfect past.
Have you ever run into an ex and wished they were just a little bit meaner? That's the entire mood of this song by the Mexican-American sister duo Ha*Ash. The singer is just minding her own business when, against all odds, she runs into a former love: "Si es tan grande esta ciudad / Hoy te tuve que encontrar" (If this city is so big / Today I had to find you). His smile immediately unlocks feelings she thought she had put away for good, and she calls him "el peor amor que he conocido / Tan perfecto que no te olvido" (the worst love I have known / So perfect that I can't forget you). It's a fantastic, dramatic line—he's the worst because he was too good to get over!
Instead of trying to be friends, she has a much better idea. She begs him to help her move on by being a terrible person. She pleads, "Piensa en mí ayúdame a odiarte" (Think of me, help me to hate you) and tells him to do cowardly things. Her main request is simple and direct: "Se un ex de verdad y trátame mal" (Be a real ex and treat me badly). It's a funny and relatable take on how hard it is to get over someone who hasn't given you a good reason to.