On this page I've found the following statement:
Taxes on actively managed funds can be considerably higher than those on index funds.
Why is that the case? Since both fund types consist of the same type of stock.
This depends on the particular index, of course. Capital gains taxes occur when stock is sold (for a profit). This occurs less frequently in an index fund: Where an active manager frequently buys and sells stocks (after all, he wants to be active :-) ), the index fund only sells stocks when the particular stock leaves the index. For an index such as the S&P 500 this does not happen that often.
The more specific the criteria of the index fund, the more often the selling of stock and thus the need to pay capital gains taxes occurs.
First, consider what causes taxes to apply to a mutual fund, index or actively managed. Dividends and capital gains are generally what will be distributed to shareholders given the nature of a mutual fund since the fund itself doesn't pay taxes. For funds held in IRAs or other tax-advantaged accounts, this isn't a concern and thus people may not have this concern for those situations which can account for a lot of investing situations as people may have 401(k)s and IRAs that hold their investments rather than taxable accounts.
Second, there can be tax-managed funds so there can be cases where a fund is managed with taxes in mind that is worth noting here as what is referenced is a "Dummies" link that is making a generalization. For taxable accounts, it may make more sense to have a tax-managed fund rather than an index fund though I'd also argue to be careful of asset allocation as to maintain a purity of style can require selling of stocks that grow too big and thus trigger capital gains,e.g. small-cap and mid-cap funds that can't hold onto the winners as they would become mid-cap and large-cap instead of representing the proper asset class.
A FUND THAT PLAYED IT SAFE--AND WAS SORRY would be a Businessweek story from 1998 of an actively managed fund that went mostly to cash and missed the rise of the stock market at that time if you want a specific example of what an actively managed fund can do that an index fund often cannot do. The index fund is to track the index and stay nearly all invested all the time.