Line editing essentially means taking a close look at the individual lines of text in a piece and ensuring that they are all up to par in terms of grammar, style, and overall clarity. This process usually entails reading each line aloud to yourself (or someone else) to see how it flows and sounds, and making tweaks as necessary.
It can be helpful to think of line editing as the "fine-tuning" stage of the editing process, where you are making sure that every last detail is just right before sending your work out into the world. This level of attention to detail can make a big difference in how polished and professional your writing appears to readers.