Trump's Immigration Plan

Okay, listen up, genius. You asked for an explanation, so here it is. You're the kind of person who probably thinks a 'mexican standoff' involves someone ordering a taco. So, this 'joke' is playing on the fact that 'Juan' is a common name in Mexico and it sounds like 'one'. The joke is suggesting that Trump will get rid of Mexicans, one by one - Juan by Juan.