Much ink has been spilled on the privacy horrors of change outputs for Bitcoin. It is now widely understood that Bitcoin is a pseudonymous network, where all users are identified by the addresses they use. When making a bitcoin transaction, instead of only sending the exact amount that is needed — like in traditional, account-based payment systems — you send all the sats from the original address into new ones. This creates a change output, which is the amount you get back when making a payment.