Selling a position at a loss lets you offset capital gains and up to $3,000 of ordinary income per year, carrying the rest forward. The benefit is a deferral, not an erasure, because your basis in the replacement holding is lower.
The wash-sale rule disallows the loss if you buy a substantially identical security within 30 days before or after the sale - and it follows you across accounts, including your IRA and your spouse's accounts. Automatic dividend reinvestment is the most common way people trigger it by accident.
Harvest into a similar-but-not-identical fund, turn off reinvestment in the position you are harvesting, and keep the paperwork.