Prep

Prep is what you do between sessions, the lonely fun (or for some, the stressful work) of getting ready for the next game. Prep involves reviewing your notes from previous sessions, thinking about what might happen in the next session, and doing stuff that you don't want to do during play.

Why prep? A few reasons:

To give you interesting stuff to say.

To speed up play, and not make everyone wait while you decide what happens, or stat up a monster, or draw a map.

To shore up your weaknesses. If you, say, always forget about the PCs' followers and let them fade into the background, maybe prep some interesting things for those followers to say or do.

To hone your craft. Some stuff is better when you have time to think about it—it's more consistent or thematic, it flows better, it builds on stuff that came up earlier, etc.

To give yourself permission to play hard and inflict badness on the PCs. If you've got a tendency to pull punches (and don't we all?), it can really help to have an impending doom established or to give your bad guys a move like "tear an arm off."

Prep only as much as you find useful and valuable. Unless you're making something for other GMs to use, there's no point in writing down details that are self-evident to you. Don't write down what Nia looks like unless you think you'll forget or it's something you specifically want to call attention to in play.

Consider prepping some or all of the following:

The ongoing, recurring problems that cause trouble for the PCs. Things that are going to get worse if left unchecked, or complicated relationships that you want to explore more in play. There's a whole chapter on prepping and using threats.