This is a supporting number, not one to study deeply, but you should know what it is because it is the denominator of EPS. It is the official share count used to split profit per share, made accurate in two ways: it includes shares that would exist if employee stock options were exercised (the honest, fuller count), and it averages the count across the year, since companies issue and buy back shares mid-year.
The practical use is a single glance at its direction over the years: falling means buybacks are concentrating your ownership, rising means dilution is thinning it.