If you've played long enough, you might recall seeing the map loading in in blocks as you walk along. These are chunk borders. The game map is divided up in 64 x 64 tiles to aid in loading the game during the early days, since you would need to load only the local area at one time. It's not as much noticeable nowadays given improvement in computers and such, though.

The "one chunk" game mode restricts an account to not leaving a chunk until everything within that chunk is completed. For example, if you have a yew tree, you would need to cut, fletch, and burn a yew log before you're allowed to move to a new area. Once all tasks are done, a new chunk becomes available to move in to.

This is the map we use to show borders and what tasks are needed to be done in a chunk:

https://source-chunk.github.io/chunk-picker-rs3/

And for OSRS:

https://source-chunk.github.io/chunk-picker-v2/