Everything that happens after your decision stops being your problem.
means In the direction a river flows, or metaphorically, the consequences and effects that follow an initial action or decision.
from Literally from river navigation: 'down' the current's direction + 'stream' the flow itself. By the 1800s, industries using water power used it directionally. Metaphorical use exploded in software and business: if you ship broken code, the QA team is downstream of your mess.
mississippi river delta — new orleans sits 500 miles downstream of memphis, catching everything upstream sends
software supply chain — npm package vulnerabilities affect millions of downstream developers
flint water crisis — lead contamination hit downstream residents in 2014, upstream decisions by officials