A sorcerer-wight has awakened in his barrow after his ancestral necklace was filched by a hobbit thief, and he has sent undead minions forth to sack the countryside and find it.
That's a plot. You (the GM) know it's a cool idea, but the players aren't going to see that until they put all the pieces together. Will that happen this game session, or four sessions down the road? What are you going to show them, now? You need a situation.
The heroes spend the night at a cheery road-side inn, but awaken in the middle of the night to find it besieged by hordes of zombies. Travelers and tavern wenches scream and run around in a panic. Perimeter defense and window-boarding ensues.
That's a situation. Players see situations immediately. It's part of the same plot above, but it's the part the players actually get to deal with, now. From their point of view it's the meat of the game. In fact it is the game, because it is what happens, not the subtext you (the GM) have in mind but haven't fully revealed yet. Ask a player about a game that happened a long time ago — the parts they remember are probably situations they were in (or really good dice rolls). They will remember defending the tavern from zombies, not the wight's stolen necklace.
GMs are very keen on plots. Nothing wrong with plots, but most plots involve finding things out over time, so players can go through game after game before they see the entire plot and understand what is going on.
All plot and no situation is player hell (and player hell is GM hell, sooner or later). That's the game where the players know something is going on, but can't really put their finger on what they are supposed to be doing. Aimless wandering ensues. On the other hand, all situation and no plot can play quite nicely for a few games, but sooner or later the players may start to wonder how it all is going to tie together.
But that's later on. To run a game, right now, you need situations, not plots:
- Your hovercraft breaks down in the deep desert, and it's a long walk to the nearest outpost.
- The sleepy coastal village where the travelers bed down turns out to be full of evil cultists.
- The superhero PCs are swarmed by reporters after false rumors surface that they have become corporate sponsors.
Those are good situations. Why is any of that happening? What does it lead to? What happens next? We'll find out later. Good situations keep the players involved, which makes them curious about the plot, even if takes a while to unfold.
If you have a situation (preferably several), you are ready to run the game. You have something to show the players, something for them to deal with. If you don't have a situation ready to go, well, it's improv time.