Ringing is what we call the small waves or shadows that usually appear around sharp corners on a print. It's often very apparent if you have text on your print as you will see what looks like shadows of the text. This happens when the print head makes a sudden direction change and the inertia of the head causes vibrations that show up in the print. To combat this there's a couple of things you can do. Simply slowing down your print speed will help lessen the effect. Decreasing the acceleration of the printer will make a big impact on the reduction of ringing without affecting print speed too heavily. You adjust this setting by going into the advanced settings on the printer itself Maintenance -> Advanced -> Motion settings. Try 3000 or 1500 and see how that effects your print.
There's also another defect that can look like ringing while it actually isn't. In some cases infill shows through to the outside layer. This can look similar to ringing. To get your print of this you must print thicker outer walls to hide the infill. Try printing at least additional outer shells over top of one another, which, with a standard nozzle, will equate to something within the ballpark of 0.8mm thick walls. But then-again, the difference of having a 50 or 300 micron per layer height can factor into things. While a smaller layer height will automatically factor into having to print more outer layers, a thicker layer height will not free you from printing multiple layers as the outside ones must create a bond to ensure a strong shell with the foundation outer layer.
Excessive temperature can also cause strange vertical lines in a print. If you feel that you are already printing "a liitle hot" try lowering the temperature slightly and see if that helps.
Yet another thing to try is to rotate your print 45 degrees on the print bed. Some users have reported that this has gotten rid of vertical lines on the print.