What does this phrase means: (nail indentations)

Drywall installers and finishers perform some or all of the following duties: Measure, cut and fit drywall sheets for installation on walls and ceilings Position and secure sheets to metal or wooden studs or joists Cut and install metal corner beads to protect exterior corners Fill joints, nail indentations, holes and cracks with joint compound using trowel and broad knife Tape over joints using taping machine and embed tape in compound Smooth out excess compound and allow coat to dry Apply successive coats of compound and sand seams and joints Fabricate and install suspended metal ceiling grids and place in panels to form acoustical and coffered ceilings.