¿Qué le dijo un techo a otro techo? (What did one roof say to the other roof?)
Te echo de menos. (I miss you.)
Words in Spanish can be put together or split apart to create two completely different words. That’s what’s happening here.
Techo means “roof.” Te echo de menos means “I miss you.” When you combine te + echo into techo, that’s wher the joke is.