What is the meaning of "bright unresolved sources"
"Unresolved" here means a source of light that appears as a point, not as a disc or a cloud.

For example, the planets can all be resolved to discs by even quite moderate telescopes. Galaxies also can be resolved, they don't appear as point sources. 

However, stars, quasars, and most asteroids cannot be resolved by amateur equipment. In the context of the LSST paper, resolved sources are "galaxies" and unresolved sources are "stars and quasars".


  The photometric measurements for resolved sources (galaxies) have to include several standard magnitudes, such as Petrosian magnitudes, as well as appropriate model magnitudes.