Framing a Scene is the core of the gameplay - an interesting scene is proposed by the player, either a completely new plot hook or something following from what's already happened. It always starts in the middle of the action and the goal is to answer a question ("How do we win this competition?")
Narrating a Slice of Life is exactly what it sounds like - you describe your witch in her daily life, doing something funny, or cute, or interesting. Other players can chime in but this isn't a plot scene. It's a little vignette for when you have a cool idea you want to get out there.
Getting a read on the academy means asking a question that establishes a new fact once answered ("Who really holds power in the student council?"). Everyone contributes to the answer and whatever is decided becomes a new fact.
What this achieves is that when it's your turn, you contribute as much as you're comfortable with. If today you're not feeling very engaged, you can just do slice-of-lifes - and if you're really itching for drama and action, go ahead and frame major scenes. It just works.