Short Interest

Short interest is the total number of shares of a security that have been sold short by investors but have not yet been covered or closed out. It is typically expressed as a percentage of shares outstanding or as a number of days to cover.

What this tells you

The number of shares currently sold short and not yet closed out. Reported to FINRA on a twice-monthly schedule and published with a lag.

What it does not tell you

It is old by the time you see it. The reporting lag means the figure describes positioning that may already have reversed. It also does not distinguish motive. A short position hedging a convertible bond, a market-maker's inventory and an outright bet against a company all appear identically. High short interest is frequently read as a signal about the company when it is often a signal about someone else's hedging.

Further reading: Wikipedia