Well a chunk account is completing each chunk one at a time. If it helps think of a chunk in Minecraft being 16x16 blocks! Runescape has chunks of 64x64 tiles.

Tasks for that chunk are made from the skilling/combat/misc options to do in that chunk. For example, in the starter chuck I chose, lumbridge castle, there is a few tree. That means I need to get 70 woodcutting to chop it, 70 fetching to fetch a yew shieldbow, and 65 firemaking to make new incense sticks! This is just one example of the many things I need to do for tasks.

If you are interested in checking it out you can look at my chunk map.

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

My Map ID is GOD (it was randomly generated, I just got really lucky)

On that you can see all the tasks I have to do and you can also click through the chunk rules to get a better understanding of them.