Short interest is the number of shares that have been sold short but have not yet been covered or closed out. An increase in short interest can signal that investors have become more bearish, while a decrease can signal they have become more bullish. In the case of Illinois Tool Works, the increase in short interest could indicate a bearish sentiment among traders. However, the fact that the company's short interest is less than its peer group average could be seen as a positive sign, as it indicates less negative sentiment compared to similar companies.