Short selling is when you sell something before you buy it (you try to sell high and then buy low later, instead of buying low and then selling high later). In order to sell short, you are required to borrow the shares that you then sell. So if I own 1,000 shares of Google (I wish), I could lend them to someone to sell. Nothing happens to my position, it is just that those shares are now allocated to that short sale. If I sell them then the short seller has to go borrow them elsewhere or buy the stock to close their position. In general the borrowing is either from brokers that hold shares for individuals or from large institution (mutual funds, insurance companies…).
However from everything that I read it appears the SEC hasn’t bothered to actually enforce this law much. There was a bunch of excitement recently when the SEC announced it would bother to enforce the law to protect a few large banks, many of whom are said to practice naked short selling but didn’t like it when that was done to their stock. As you can see, this does make the SEC look pretty bad, when they chose to enforce a law, not in all circumstances, but only to protect a few of those who actually take advantage of the SEC’s failure to enforce the law to make money.
Some people find the whole concept of short selling bad since it is based on making money on stock price declines. I don’t feel that way and believe it can help the market. But it requires regulators that actually do their jobs and enforce laws. A favorite tacit of those who seek to keep open special ways for themselves to benefit from abusing the system is to try and make things seem complex. The recent SEC order saying they would enforce the intent of the law to protect a few powerful banks from the behavior many (or most) practice themselves for years shows that it isn’t that complicated.
Adding the decision not to enforce the requirement to borrow shares to their recent decision to eliminate the requirement that short sales take place on down ticks in price (a measure put in after the 1929 stock market crash to not have short sellers accelerate market declines and insight panic seems like a really bad combination).