You specify a point within casting distance, and the smooth grease covers a 10-foot square area centered on that point. For the duration of the spell. The area becomes difficult terrain.
Grease 5e
level: 1st
Classes: Wizard
Casting Time: Action
Range: 60 feet
Components: V, , S, M
Duration: 1 minute
When grease appears, each creature standing within its range must make an Agility Immunity, failing which it falls to the ground. A creature that enters the area or ends its turn in the area must also make that Agility Immunity, failing which it falls to a fall.