This shows the percentage of total Common Shares Outstanding which are freely floated on the stock exchange.
Free Float = Total Shares - Treasury Stocks - Shares held by Strategic Entities). Strategic entities means the shares held by Government Agencies, Corporations, Holding Companies and/or individuals. This is where the shares are held with the intention of gaining market share and/or having control over the company, the idea being that, generally, strategic investors do not primarily intend to earn short term gains from the shares through an increase in share price unlike, say, investment management companies.
Some shares are very tightly held by founding shareholders. As they are unlikely to be sold the number of shares outstanding is not a very clear representation of how much stock is actually available to be liquidly traded on the stock market. The Free Float is a better representation although some of the shares 'freely floated' could be held just as tightly by institutional or private shareholders as founders.
A good rule of thumb from an investor point of view is whether the directors of the company own or control more than 50% of the shares.
For more on this, see this Help article.