Developers want to ship fast and operations wants stability, and arguments about it go nowhere without a shared number. An error budget is the downtime your SLO allows, 1 - SLO, which is about 43 minutes a month at 99.9%. Use it to decide when to keep shipping and when to freeze risky releases, and alert on how fast you are burning it. An unspent budget is not a win; it often means the team is moving too slowly or the SLO is too loose.