Stating the obvious it means exactly what it says.

If you have say A1 in file 1 that is named say 'myname' and in file 2 you also have a name 'myname' which may or may not refer to A1, Excel need to know which name should, be used. When you copy or move a sheet all names transfer with the copy hence there has to be a resolution when merging with a file that already contains the same name whether the name (and hence cell) on the destination file should be used rather than the name/cell on the original sheet.