Fix it in post means, if you can’t get things right while you’re filming, then get some computer expert to fix it later in post-production.
And what most people don’t realise is, it actually takes a lot of experience to know when it’s best to fix it in post. More often than not, if you can fix things at the time you take the shot, then not only will be it less work overall, but you’ll get a better result too.
A lot of still-photographers take their shots like someone who can always fix things in post. While programs like Photoshop are amazing, there’s a limit to what they can do. And why would you want to do all that fixing up afterwards if it can be avoided by just taking your shot from a different angle, for example? Pro photographers will walk around a bit and see things from different angles and consider what’s in the background and shift things out of the way and only when all the problems are fixed, do they then start taking their shots.
So when you look through the viewfinder, ask yourself if there’s something you can do to fix in ‘pre’.