Shiny "rolls" means how many times the game calls on RNG to potentially determine shininess. You can think of the base shiny rate as if you had a 4096-sided die with 4095 faces that say "not shiny" and 1 face that says "shiny". The number of rolls means how many dice you roll when a Pokémon is generated, and if at least one of them lands on a "shiny" face, your Pokémon is shiny. So for example, the Shiny Charm adds 3 rolls, meaning you roll 4 times instead of just 1 time, which is approximately a 4/4096 aka a 1/1024 chance. It's actually a little bit less—1/1024.38—because there's a very small chance that 2 or more dice landed on the "shiny" face, but we usually just say 1/1024 because it's simpler.
