As a game master, a lot of weight rests on your shoulders. You need to preplan some stories and events to give life and motivation to the world. That world needs to feel larger than the players but also needs to feel accessible. You have to have interesting and fun events happen, both expected and unexpected. You have to be able to improvise or completely alter your plans if the players do something fair but unpredictable. But if you have too little planned or have no story with your events, everything can lose its gravity and feel unfulfilling. Plus you have to come up with names for NPCs, taverns, towns, guilds, groups, religions and maybe even monsters and deities.