The resolution of an image is a measure of its sharpness, or how small a detail can be resolved.
In a (computer) printer, the resolution is measured in pixels-, dots- or lines-per-inch (dpi,lpi) - or, less commonly, per millimetre.
In film photography, resolution can be measured in terms of the maximum number lines per millimetre (or per inch) that can be produced.
Resolution can be measured for an individual component, such as the film or image sensor, or can be for a whole optical system - e.g. the combination of a particular lens and its mount and film and film transport. The mount and transport can have a small effect on the resolution, as keeping the film flat and perpendicular to the optical axis is important for consistent focus across the image.