I thought this topic would be good to cover today as it was something I personally had to deal with this very weekend. Keeping a story focused means avoiding anything (majorly) extraneous to your plot. It’s that whole thing about not throwing everything and the kitchen sink into your project in the hope that something works. You may also refer to the analogy of throwing pancakes at the wall to see what sticks (although in that case it’s better if none of it sticks, surely?)
Let me start off with my own little dilemma as an example of this in practice.
In my story, I have two main factions who tolerate each other right now. Later in the story, they will be at each other’s throats. The trap I very nearly fell into was to add a third faction, an army of foreigners, to come in and provide my story’s big conflict. What I later realised was that I already had a great, meaningful conflict built up between my two original factions. To add a third would be to rob my story of its core conflict and simply muddy the waters with mindless ‘epicness’.