Converging verticals is when something you know is upright appears to bend together and get thinner in your picture. it can look really cool as an effect - but what if you don't want it?
You can buy a specialist lens called a shift lens which you can adjust to straighten converging verticals and you can straighten them with software, but there straighter you can get them in camera when you take the photo - the easier it is to tweak them later.
Convergence is caused by the camera's sensor not being parallel to whatever it is you're photographing. It often isn't possible to get completely parallel and that's why software is so brilliant. As you look up or down onto something the distance between it and your camera sensor varies from top to bottom - causing converging verticals.
If you can move back and extend your focal length so the angles decrease and the convergence decreases along with it. (need to know more about focal lengths and what they can do? come see us at www.photography...)
For this example I chose the spot very carefully so I could move back - but the principal remains the same...