f/1.8, which means the shutter of the camera can open very wide, letting in a lot of light but at the same time giving a narrow "depth of field", so that someone's face can be in sharp focus but the background be entirely blurred.
Well, an f/1.8 lens wide open allows you to maximise shutter speed, so presumably you're talking about very low light/slow film situations. (N.B. you have a typo, s/shutter/aperture/).