Average maturity is the average length of time until the bonds or other debtsecurities in a fund or portfolio reach their maturity dates. Funds almost always weight each holding by its share of the portfolio, so the figure is usually published as weighted average maturity (WAM), which is explained in full, with the formula, on that page.
The longer the average maturity, the more the fund's value tends to move when rates change, a sensitivity covered under interest rate risk. It is one of the figures shown on a fund fact sheet for a bond fund.