The 12b-1 fee covers distribution & marketing costs incurred by the fund, including commissions paid to brokers.
12b-1 fees are included in the calculation of the Expense Ratio for the mutual fund and are required to be disclosed in the Prospectus for the fund. These fees are charged as a percentage of the assets in the fund.
All fees reduce your overall annual returns and can vary widely from one fund to another, so prudent investors research fees before investing in a fund.
Some funds temporarily reduce or eliminate 12b-1 fees as a way to lower fees & show improved returns, but be careful because these fee reductions will expire at some point & reduce your returns accordingly.