Critic speak is tough, but we've got you covered.
Quote :"Note on the Treatment of the Text Adopted in This Edition," in The Works of Thomas Nashe, Vol. 1
The spelling of the copy-text, by which, here and throughout the book, I mean the text used in each particular case as the basis of mine, [is] followed exactly except as regards evident misprints.
When McKerrow says "copy-text," he's referring to the manuscript a scholar is basing his or her own edited edition on. Let's say you're editing Shakespeare's Hamlet. Hamlet exists in three versions, which are found in documents known as the First Quarto, the Second Quarto, and the Folio. Textual Critics have to decide which of these three versions of Hamlet they're going to use as a copy-text, or the basis of their own edited edition of Hamlet.
The copy-text has to be the most authoritative version of Hamlet available. If you decided that the Folio version of Hamlet was the most complete and authoritative version of the play, then you would use that version as your copy-text.