A return of principal (or return of capital) reduces your basis in your mutual fund shares. Unlike a dividend or a capital gain distribution, a return of capital is a return of part of your investment (cost). However, basis cannot be reduced below zero. Once your basis reaches zero, any return of principal is capital gain and must be reported on Form 1040 Schedule D (PDF), Capital Gains and Losses.

References:

• Publication 564, Mutual Fund Distributions

Source: www.irs.gov