When portfolio managers of mutual funds buy and sell stocks in the market to alter the fund portfolio, it is referred to as 'turnover'.
While not necessarily a bad thing at the outset, turnover generates transaction fees and potential taxable events for fund investors.
In general, lower fund turnover rates signal higher quality - but the turnover will depend on the type of fund and its stated investment strategy.
Growth funds tend to have higher turnover rates as they invest more actively.
On the other end of the spectrum, index funds should have very low turnover rates as they only trade when the index they're tracking changes composition.