Tartamudear means 'to stutter' or 'to stammer'. It's a beautifully descriptive verb that perfectly captures the feeling of being tongue-tied and nervous around someone you like.
In the song, Beret pleads for his love not to leave when he gets so flustered that he starts to stutter and can't get his words out. It's a very human and vulnerable moment that makes the word especially memorable.
This song is a desperate plea from someone who can't believe they're finally with the person they've been dreaming of. The singer, Spain's Beret, kicks things off by remembering a past summer and "cada beso que nunca pasó" (every kiss that never happened), a memory that haunts him like a ghost. It seems like things ended suddenly, going "del calor al frío" (from hot to cold), but now, unbelievably, this person is right next to him.
He's so terrified of this perfect moment ending. The chorus is just him begging, "Porfa, no te vayas cuando salga el sol" (Please, don't go when the sun comes up). He's worried he'll mess it up by being clumsy or reckless. He's so nervous he can't even speak properly, admitting that when he tries to talk, he just stutters and tells her "na-na-nada" (no-no-nothing). It's a super relatable feeling of being so overwhelmed with happiness and anxiety that you just hope the other person doesn't leave when you show your imperfect, nervous self.