KiCad 5 now comes with global and project local symbol library tables (like the footprint did already in version 4)
For some reason the symbol library table side offers more options than the footprint side of things. With the footprint side you only get a notice that the default table got copied if there was no table in your users config directory.
So what do the 3 options for the symbol side mean?
Copy default global symbol library table:
This copies the library table that comes with your installation to your users config directory. This library table is the one that is maintained by the maintenance team of the official library. It resides inside the github repo for the symbol libraries. (Edit: The maintained table is inside the repo. The one that will be copied is the one that is inside found in your system directory.)
Copy custom library table:
This enables the “select global library table” option on the bottom of the dialog. Clicking that link will allow you to select an already existing library table. This is useful for users who already ran kicad 5 and have a custom library table setup. (Or if they have setup the library table using a text editor or some kind of script.)
Create empty library table
This like the name states sets up kicad without any libraries. It is useful if you do not yet have your own library table but do not want to add all libs of the official lib. (You either want to exclusively run your own libs or you want to select which official libs to add.)
No mater what option you choose you can always edit the lib table later on. Either with the kicad internal library managers or with a text editor.