Ability Chunks are items that can help you reroll certain abilities on your favourite pieces of gear. This allows you to craft that equipment so that it best suits your playing style.
There are several ways that Ability Chunks can be obtained but the most reliable method of farming is by scrubbing your unwanted gear by talking to Murch who is located next to the online lobby building. Ability Chunks can also be obtained as rewards through Salmon Run but the drop rate is low.