A subgiant star is brighter than a normal main sequence star of the same spectral type, but not as bright as a giant star. They are post-main-sequence stars undergoing the same type of increased luminosity and radius as a giant star, but less so. They are generally slightly more than solar mass.
Some somewhat-low mass stars (e.g., the Sun) have a phase in their stellar evolution when they fit this fit this category. The phase, termed the subgiant branch (SGB), is their time transitioning from the main sequence to the red-giant branch. On the H-R diagram (HRD), they fall in a relatively empty region (due to the short lifetime of this phase) termed the Hertzsprung gap (HG) and the stars are sometimes referred to as HG stars.
The stellar luminosity class IV indicates a subgiant star.