Inbound=outbound
The principle that a contribution to an open project comes in under the same licence the project goes out under — no separate agreement required.
The phrase was popularised around 2011 by Red Hat lawyer Richard Fontana as an alternative to CLAs. The reasoning is that someone sending a patch to an MIT project presumably means it to be under MIT, and that this is enough. Apache License 2.0 already spells it out in section 5: contributions intentionally submitted fall under the licence's terms unless stated otherwise.
GitHub made it the default for the whole platform in 2017, adding a clause to its terms of service saying contributions to a public repository are licensed under that repository's licence. The drawback, from a project owner's point of view, is that nobody has handed over any extra rights. A later relicensing means asking every contributor for permission — which is precisely why contributors like the model.