This is a very Mexican expression that means you finally realized or understood something.
For example: "Hoy me cayó el veinte de que te extraño" - Today I realized that I missed you.
This expression actual originates from using public telephones in Mexico. There's a 20 centavos moneda (coin) that people use for pay phones. That's where the veinte comes in.
Now, if you've ever used public phones with any frequency (tú sabes, back when you could find one on every corner) you know that sometimes you put in your coins, and they don't "catch". After 3 or 4 attempts, the money finally "catches" and you can make your call. And that's where the rest of the phrase "cayó el veinte " comes from. The money finally catches (or falls, if you prefer a more literal translation) and you can make your phone call. Make sense?
Hmm, pay phones are getting harder and harder to find, and me cayó el veinte that I'm giving away my age by admitting to the fact that I was around when public phones were on every corner in the US!