Copyediting is the process of checking and reviewing a manuscript for inconsistencies, repetition and errors in grammar and punctuation.
A copyeditor is not a glorified spell checker as many believe but a partner in your journey of getting your book published.
A copyeditor will ensure that your manuscript tells the best story possible by keeping the big picture in mind and looking closely at all small details.
As a writer you are unable to see where you have missed out in terms of the plot, flow of story and grammar among other things. You will need a second pair of eyes before you even begin to finalise your manuscript for publishing.
Many self published authors are opting out of this crucial step only to regret their impulsive actions.
A copyeditor:
Checks for and corrects errors in grammar, spelling, syntax, and punctuation.
Checks for technical consistency in spelling, capitalization, font usage, numerals, hyphenation. For example, is it e-mail on page 26 and email on page 143? Or do you use both British and American English spelling variations interchangeably, such as favourite vs. favorite?
Checks for continuity errors and makes sure that all loose ends are tied.
Checks for factually incorrect statements. This is a necessary part of the copyediting process for non-fiction manuscripts, such as historical pieces and memoirs. The copyeditor must check if the facts in your manuscript are accurate and if the names and dates are correct.
Checks for potential legal liability.
Checks for inconsistency within the story. This includes character description, plot points, and setting. Does each character stay true to his own description throughout the story? Are there conflicting descriptions of the house? For example, have you described the setting as “a yellow brick home” on one page but “a weathered wooden home” on another page?
The copyeditor’s job is not just to check grammar and spelling. They ensure that each element of your story is consistent, cohesive, and complete.
Source: https://nybookeditors.com/2016/05/whats-the-difference-between-copyediting-and-proofreading/