Cosquillas literally means "tickles". It's a fun and common word, but Ivan Cornejo uses it in a very unique and poetic way in this song.
He sings about someone who gives him "cosquillas en la mente" (tickles in the mind). This beautiful metaphor describes the feeling of constantly thinking about someone in a happy, exciting way. It's as if the memory of them is a pleasant, persistent thought that you just can't shake.
This song is one of the most dramatic, funny, and relatable breakup songs you'll ever hear. Ivan Cornejo is completely falling apart after being dumped, and he's not afraid to admit it. He says his ex made a "fatal" mistake by leaving him, and now he feels useless, like an "unwashed dish" (un traste sin lavar). His life is a mess, and he's so miserable that he says, without exaggerating, he wishes he was back in his mom's belly (en la panza de mi 'amá). It's a hilarious and over-the-top way to say you're having a bad time.
The whole song is a desperate plea for his ex to come back. He begs her to "tell me you'll never fall in love with someone else" (dime que jamás te vas a enamorar de alguien más) and compares himself to a dog getting excited to see her. But the best part is when he reveals the real reason he can't move on. He's tried dating other people, but he has to admit that "nobody makes my cereal like you do" (nadie me lo hace como tú el cereal). It's such a specific and silly detail that it makes his huge heartbreak feel completely real.