Line twists occur when a canopy opens with the jumper rotating relative to it, twisting the suspension lines together. The canopy may be perfectly good, but steering is unavailable until the twists are cleared.
Common, and usually minor
On a large, docile canopy, line twists are a routine annoyance that clears in seconds. On a small, heavily loaded canopy they are more serious, because the canopy may begin to spin and lose altitude quickly while the jumper is unable to control it.
This difference is one of the standard arguments against downsizing quickly — the same event has very different consequences depending on the wing.
What to do about them is trained
Whether to work on twists or go straight to emergency procedures is an altitude-dependent judgement taught by instructors and specific to your canopy. It is not a decision to make from a web page in the moment.
These pages explain what things are so the language makes sense. They are not instruction and cannot substitute for it — skydiving is taught in person by rated instructors at a drop zone, and anything involving equipment handling or emergency procedures belongs there rather than on a web page.