The "float" of a company (FLOAT_SHARES) is the number of shares that are available to be bought by the general public, as compared to restricted shares, which are owned by insiders / employees of the company and are not allowed to sell for a certain period of time (said to be in "lock up"). For instance, AAPL has over 16 billion float shares.
The floating shares number is calculated by subtracting the number of restricted shares from the total number of outstanding shares of the company. Example: suppose a company has 5 million shares, 2 million of which are owned by company insiders and they can't sell at this time. So, the rest of the 3 million shares can be bought and sold by the general public.