You wound that commented out orbital nav aid? Note that it will go away with the UI rewrite, but we also want to introduce a proper orbit display into the HUD to aid navigation.
Tangential speed means your speed parallel to the ground, let’s say. For any given height, a certain speed in a certain direction means a certain shape of orbit (another simplification).
But I’d suggest don’t worry much about it, use the orbital display instead for orbital stuff. First you should fly to a good height (100km for Earth is about the minimum), then rotate your ship parallel to the ground. Side and down views are helpful for that, or the HUD indicators. After that start accelerating in manual flight (W) and without releasing the button, press F2 or the Orbital map button on the top. This will open up the map, but the engines won’t turn off, even if you release the accel button. (Which is actually a bug, but a useful one) Now you can watch how your orbit changes in real time. And when you are at the correct shape, you can go back to the Worldview (F1) and your engines will be turned off, since you no longer pressing the button.
There’s also an orbit planner function you can play with, setting up changes in speed on three axes. It will display a marker and needed deltav, burn time on the HUD, if you set up something there. Kinda like in KSP.