A quick example
Maria had a fund that was down $6,000. She sold it and, the same day, bought a similar fund so her money stayed invested. That $6,000 loss lowered her taxable income. Because she did not sell anything else at a profit this year, about $3,000 of it cut her income now, saving her around $660, and the rest was banked to lower her tax bills in future years.
Example for illustration. Your numbers depend on your own situation.