A slugline is the one-sentence description of the location and time of the scene.

It begins with INT/EXT -- INT for 'interior' and EXT for 'exterior'. Is your scene inside or outside?

Then you describe the primary location of the scene -- a PARKING LOT, an APARTMENT, a GRAVEYARD, wherever you'd like.

Finally, you include the time -- DAY or NIGHT. Or sometimes DAWN or DUSK, if you want to get a little more specific.

The result is something like this:

INT. HUT - DAY

EXT. CAR (MOVING) - NIGHT

EXT. HOSPITAL - DAWN

... and so on.