If you don't know what this phrase means, it's something commonly said by filmmakers who know that there's something not quite right with a scene, but instead of filming it again and again and again, they decided to move on to something else and fix it later, during editing.
(Or, at least, it's commonly said by people pretending to be filmmakers in TV shows and movies.)
It's a helpful phrase in that it means: instead of keeping all the actors, directors, camera operators, grips, gaffers, sound ops, extras, and any of the other long list of people who work on movies around, they get to send everyone home and deal with it later. They realize that they won't be able to fix anything right now, and that to keep trying would be an exercise in futility (and possibly also result in over-time).
This phrase is also something that I've been muttering to myself while writing. Stuck on the wording in a sentence? Story's not working out the way you've intended? Feel like you forgot to mention something in an earlier paragraph? Fix it in post.
See, lately I've been doing a lot of first drafts, trying to build up my story-stash, and some of these drafts are definitely works-in-progress. However, instead of fixating on a problem, like I used to do in the past, and wasting time trying to get that one thing to sound absolutely perfect, I've been choosing to deal with it later. And (so far) it's working for me.
Aside from removing a lot of perfection-related stress, it gives me a chance to move on and keep writing. If I fixate on something for too long I might get frustrated and decide to walk away from the project, and then I'll forget everything and never remember where I was going with it. (Note to self: always write plot notes, even if you think you'll remember it – because most of the time you never will.)
I recently finished writing a short story, and while I was writing the end, I was aware that this wasn't the ending I wanted (nor was it was the one I needed). The story needed something with a punch, but at the moment I was writing a lot of exposition. However, instead of stopping and going back and trying to fix everything, I continued writing the exposition and reached the “end” of the story. I knew that Future-Me would find a way to shuffle it all around and work that exposition into the story in order to achieve the 'punchy' ending I desired. And so, instead of ending up with an unfinished story, I had a first draft that I could work on.
So go ahead and write. Write, even though you know it's wrong. Get it all down and worry about polishing it up later.
Fix it in post.