If the tags are anything like DWM or Awesome, they're like traditional workspaces like you'd find in Gnome or KDE, but with the option to display multiple workspaces at the same time.

Example:
You have a "coding" tag that contains your editor and terminal, and a "browsing" tag that contains Firefox and Chrome. You can display the windows in just "coding" or "browsing," or you can display all windows from both tags at the same time.