Every game of OSR+, whether it's a one-shot or a fully fledged campaign that will last many sessions, starts with a premise. A premise is the basis of your agenda as a GM. Everything you create for the adventure must serve this premise, as it's a promise you make to yourself and your players about what the adventure entails.
The simpler the premise, the better. It needs to establish a genre, a tone, and a vague idea as to what the PCs will be doing in the fiction. Let's look at several premises from official worlds of OSR+:
Each premise above sets an expectation of the sort of game to be played: silly vs. grimdark vs. epic. Each describes a general motivation for PCs in the fiction: solving a mystery in college, surviving aboard a battle train, and restoring an elven queen to power. Each paints a picture of the boundaries of the fiction with a broad brush: a magical college, a violent battle train, a whole medieval fantasy world.
A premise must evoke a specific vision of the game that's easy for everyone to grasp. Use well-known examples from speculative fiction to compare your premise to, as that will help your players relate. In fact, there's no shame in outright stealing a premise from a movie, book, or video game you like. After all, if you want to play the time-traveling cops from Timecop, well then, that's your premise!
The Promise of the Premise
The purpose of sharing a premise with your players is to get them excited about what's in store for them, and to make sure everybody's on board with what the fiction demands of them.
We wouldn't want a drunken, college-aged frat wizard from Magic University playing at hijinks in A Quest of Queens campaign, and we wouldn't want a heroic knight from A Quest of Queens monologuing about the virtue of fighting insurmountable odds in a Magic University campaign.
The promise of the premise, in many ways, is a social contract you make with everyone at the table. Having a premise everybody agrees with gives you the authority to steer players away from character concepts and story hooks that don't mesh well with it in session zero, and works as a reminder that any content you introduce into the adventure needs to deliver on that premise as you develop future sessions.