Definition
A worktree is a working directory in Git with a branch checked out. With the git worktree command, one repository can have several branches checked out in different directories at the same time.
How it works
Each worktree has its own branch and files, independent of the others, while sharing the same object store and refs; git worktree add <path> <branch> creates one, with no need to clone the repository again.
Usage
It suits parallel work: editing code on two branches at once, keeping a directory that builds continuously, or giving parallel agents separate working directories that never interfere.
Related and confusable terms
A fork differs in scope: forking copies the repository under another account or project as an independent one, while a worktree always stays inside the same repository—it just opens extra checked-out directories.