Personal chunk loaders keep a single chunk loaded into memory, even when you're not standing in that particular chunk. To activate a loader, place an Iron Block inside the chunk you want to keep loaded, and right-click it with a Blaze Rod.
These can be hugely beneficial when you want to spread your base out further than a few chunks, but also want machines to keep running when you aren't close to them.
Note that these loaders do not remain active when you log off.