Settlements are reports sent by the channel that contains information about any order related fees, commissions and taxes.
Some channels like Amazon end eBay have the settlements as a part of their feed. Those get imported into SellerCloud automatically on a regular intervals.
Other channels like Walmart Marketplace does not include the settlements in their need and they need to be manually imported into SellerCloud.
More info on different channel's settlements can be found on those pages: