Used by programmers. When a programmer writes a patch (like everything in software, made from bits) for a big project, he would want it to be merged with the project as soon as possible. If the patch is not merged and other patches are applied to the project, the patch may not apply anymore or conflict with patches added between the time the patch was written and the present.
For example, if I've written a patch for Firefox 5 years ago, I won't be able to use it to patch the current version of Firefox.
For non-geeks, compare it to a car. You and me are building a car (big project). I've made an engine (the original patch) but haven't installed it in our car yet because I didn't have the time. While I'm trying to find time to install the engine, you install some bicycle pedals (a conflicting patch). My engine won't fit anymore, it now suffers from bitrot.
I want to get this patch upstream
, so that it doesn't suffer from bitrot.