What does map<C-\>^] mean?

The <C-\> part is, as you can guess, pressing the control key while pressing the backslash.

^] can be entered by pressing while pressing the control key.To write this control character directly to the script, Vim can enter it by pressing the <C-v><C-]>> key in insert mode.
If you really want to set it up, it would be easier to understand if you set it up using the < notation as follows:

noremap<C-\>C-]>:GtagsCursor<CR>

**At first, I mistook it for <C-[> and said it was an escape key, but I corrected it because it was wrong.