On Pipedream, a "fork" refers to a copy of an existing workflow. When you find a public workflow that you want to use yourself, you fork it to make a copy of the workflow that lives in your account.
When you find the workflow that you'd like to fork, click the big green Fork button near the top-right:
If you're not signed into your Pipedream account, or haven't yet signed up, you'll be prompted to.
After forking, you'll now have a copy of the same workflow in your account, ready to activate and send data to. You'll also see the workflow you originally forked to create this copy in the top-right: