This refers to very apparent lines on the side of your print that should not be there (like in the image shown), not the standard layer lines of your print.
Signs of lines on the side of your prints:
You will be able to see off lines on the side of your print. The lines may be vertical and possibly even wobbly, like in the image shown.
Explanation:
Besides trying to print way too fast (incl. too high acceleration and jerk settings), the root cause of these 3D printing errors are often of mechanical nature.
How to fix:
Check Z-axis assembly, it may be bent or misaligned.
There may be lose pulleys, make sure they are tightened.
Check belt tension of your X & Y axes.
Friction on the axes may be preventing smooth movement over the print. Clean and lubricate the axes.
Make sure there are no big variations in temperature during printing. No sudden increase in fans or drop in room temperature/draughts.
Inconsistent extrusion may cause this issue. See [Under Extrusion] and/or [Over Extrusion].
Lower your printing speed and check acceleration and jerk settings, these may also be too high.
Source: https://www.3dsourced.com/rigid-ink/ultimate-3d-printing-troubleshooting-guide/#lines-on-side