It's an extension for github.com/cli/cli that adds some stuff for managing git worktrees.

commands to create and manage git worktrees
Usage:
worktree [flags]
worktree [command]
Available Commands:
completion Generate the autocompletion script for the specified shell
help Help about any command
pr worktree from PR
Flags:
-h, --help helpfor worktree
Use "worktree [command] --help"for more information about a command.