RNG abuse is not something easily explained - for the full explanation, check
Smogon.
In lighter terms, though, RNG (Random Number Generator) abuse involves finding out the seed the game currently has in use for Pokémon generation and that, along with the ID and Secret ID of your game, lets you predict at which point you can get a specific Pokémon (certain IVs, Shiny or not, nature or a combination of these factors) - with some broken things in the game, you can advance said moment to your present time.
In other words, I can get a Shiny very quickly - it's not hacked per se, but it's an exploit that will guarantee the result I want (in this case, a Shiny), if I have the procedure done correctly.
It's also explained on the main FAQ of the WiFi trade section, I believe. :)