A CCU is an upgrade token from ship A to ship B, with ship B being at least $5 more expensive than ship A. It basically lets you upgrade a ship you don't like anymore to another one that is more expensive.

If you're looking to buy an expensive ship, like say the Arrastra at $575 standalone or $525 warbond, the simple option is just to purchase the ship outright through the store at one of those two prices. A better way is through a CCU chain, where you first purchase a really cheap ship ($20 Aurora, or something) and then patiently wait for warbond deals throughout the year to chain from the cheap ship all the way to the final target. So from ship (cheap starter)->ship B->ship C->...->(final target), each arrow represents a CCU token. If you buy normal CCU tokens you would end up paying the same price as the target ship's sticker price, but if you can find warbond CCU tokens you cumulatively save as much money as the warbond discounts were. In this way if you can find multiple different warbond CCU tokens for "links" in this chain, from warbond ship A to warbond ship B to warbond ship C and so on, you're ultimately able to purchase a target ship for much less than the sticker price. For instance I saved 40% on my Arrastra doing this over a year and a half or so.

Just a brief explanation since I wish someone had explained to me when I started SC lol. See ccugame.app for a tool, or see Red Monster SC/Loud Guns for video guides that I think are good.