differentiator is a website that lets you automate your Manifold Markets account. Right now, it only supports creating limit markets that expire after a certain amount of time. More features are planned, though, like automatic market resolution and arbitrage.
To use differentiator, you need to get your Manifold Markets API key, which you can do by going to your profile page, clicking the pencil icon, and scrolling down to where it says "API key." Once you have an account, you can go to the trade page to see previous trades and create new ones. Currently, the only type of trade you can make are expiring limit orders.
There are a bunch of trading bots on Manifold named after the derivatives of position in physics: Position, Acceleration, Velocity, Jerk, etc.
differentiator lets you utilize automated functionality on your own account, so it was only logical to name it after the operation used to relate all of these trading bots: differentiation. If you haven't studied calculus, this won't make sense, sorry.