There are many different types of editing, but line editing is specifically focused on improving the flow and clarity of the writing on a line-by-line basis. This can involve making sure that each sentence is well-constructed and flows smoothly into the next, ensuring that the overall argument is clear and fixing any issues with grammar or syntax.
Line editing is a vital part of the editing process, and can make a big difference in the quality of your writing. If you're looking to improve your writing, consider hiring a line editor to help you fine-tune your work.