You are watching an action movie, and during the fight scene, the two sides shoot and shoot and shoot some more. And while you are engrossed in the action, somewhere in the back of your mind you are wondering “Shouldn’t they run out of bullets or at least need to reload?”
Just like watching that action scene momentarily jars you out of the story, many things in your novel can have the same effect on your reader. And there is nothing worse than reading and enjoying a book when you come across something that pulls you out of the story. It can be anything. It can be that your hero traveled 200 miles in one day on horseback or that a camp side stew was made in the time it took to set up camp. (Just for the record, a horse can travel about 30 miles in a day, and a rabbit stew takes a minimum of two hours to cook and that doesn’t include prep time.)
Sorry, I am a fantasy writer so my examples are based on situations that will come up in my storytelling but this holds true for all writers. For example, you don’t want to have your character fly across the country in just four hours when it takes at least seven on a commercial airliner or travel from Los Angeles to San Francisco in a two hours when it takes six.
Creating a spreadsheet or map to keep track of distances between locations in your novel can help you determine how far of a trip it is and to be consistent. Your reader will notice if it took them a week to get to the capital but only two days to return home.
Another area in the fantasy realm that you need to make believable is magic. You need to clearly define (at least for yourself) what can and cannot be done with magic. There must be limits on magic otherwise the person using magic would always win and there would be no conflict in your story. There are countless ways to limit magic: power is drawn from magical lines through the ground, and if you aren’t near one then you have no magic; magic is based on knowing spells so you are limited by your knowledge; magic makes a sound other sorcerers can hear and thus can find you, and the list goes on and on. Decide which rules you want to use and then make sure you stick to them in your story.
Being consistent with your details whether they are about magic or something as simple as what your character is wearing (a blue shirt one moment and red one the next) are very important in allowing your reader to be immersed in your make-believe world. And when it comes to areas that you aren’t knowledgeable about (perhaps traveling a long distance on a horse), then make sure you do the research so you can accurately portray the scene in your novel and not jar your reader out of the story with something as ridiculous as a gun that never runs out of bullets.