ImageResolution is an option for Export and Display which specifies at what resolution bitmap images should be rendered.
ImageResolution->r specifies that a bitmap should be rendered at a resolution of r dpi.
ImageResolution is relevant only for bitmap graphics formats such as "TIFF", and not for resolution-independent formats such as "EPS".
The default setting ImageResolution->Automatic typically uses a resolution of 72 dpi for bitmap graphics formats.
See The Mathematica Book: Section 2.9.19.
See also: ImageSize.