To reattach means to attach something again after it has come loose or been removed. When a button falls off your shirt, you reattach it by sewing it back on. When a bookmark slips out of your book, you reattach it by placing it back between the pages.

Surgeons sometimes perform remarkable operations to reattach severed fingers or limbs, reconnecting tiny blood vessels and nerves so the body part can work again. Mechanics reattach car parts during repairs. Dentists can sometimes reattach a tooth that's been knocked out if you get to them quickly enough.

You'll see this word used for both physical objects and digital things. You might need to reattach a file to an email if it didn't go through the first time, or reattach a loose LEGO piece to your creation. The word emphasizes that something was connected before, came apart, and now needs to be connected again.