Feo literally translates to "ugly", but in this song, the word is reclaimed and used with a sense of pride. It's the central theme and title of the track.
The chorus states, "todas las mamichulas tienen su feo" (all the hot girls have their ugly guy), suggesting a 'beauty and the beast' dynamic. The singer embraces being the 'feo' for his partner, turning a negative adjective into a term of endearment and making it a very memorable and intriguing word.
In "TU FEO," Lenny Tavarez paints himself as a guy who knows he's messed up in the past. He calls himself a "fugitivo" [fugitive] from karma, admitting that he knows people eventually pay for their mistakes. But his consequence isn't bad at all, it's meeting an incredible woman who he thinks is an "ángel sin alas" [angel without wings]. He's so completely captivated that he doesn't care if she's lying to him, singing, "aunque digas mentiras... Yo te la creo" [even if you tell lies... I believe you].
The whole idea of the song is in the chorus. He says, "todas las mamichulas tienen su feo" [all the hot girls have their ugly one], and he's more than happy to be hers. It's a funny and confident way of saying he feels like he's won the lottery. He's so proud to be with her that he tells her, "si tú quieres me coges de trofeo" [if you want, you can take me as a trophy]. He's been hurt before, but for her, he's willing to be totally vulnerable, even saying he'd take a bullet for her without a bulletproof vest.