Portfolio Turnover Rate (PTR) measures how frequently a mutual fund buys and sells securities within its portfolio. It's calculated by dividing the value of total transactions (either buys or sells, whichever is lower) by the average value of the fund's assets. A high PTR indicates active trading, while a low PTR suggests a more buy-and-hold approach. Portfolio Turnover Rate can impact a fund's transaction costs and potentially its tax efficiency.