In Medias Res

It means, "in the middle of things". It is a literary technique where you start your story in the middle of things. It is a method that comes with pros and cons.

The positive aspect is that it skips the "boring parts". Sometimes people don't want to know why things are happening; they just want to know what's happening and they'll figure out the backstory as they go along. More importantly, sometimes they don't need to know why things are going on. If you have a story about an unwilling hero coming to terms with his personal problems in order to lead a small army into an epic battle, then the reasons for the war as a whole are completely irrelevant to the story. The story is about a person, not politics.

The problem comes with disorienting the reader. There is no introduction, no setup; you're right in the middle of things. People have to figure out the backstory as they also continue forward in time. It makes more work for the reader (which readers don't always want).

However, there is another aspect to in medias res to consider. The only way to use the technique is to have a fully-developed world and characters. They ignore the camera because they're already in the middle of things when we start watching. I think that when you are building a new world, you should see if you could start your story in medias res. If you can't, then figure out what's actually going on in your world. If you can, then feel free to start anywhere you want.