Player breifly seeing game before GUIs appear (?)

How would I fix players breifly seeing the game before the GUIs appear? I am making a loading screen, and loading screen transitions between games.

For the game transitions I used:

game:GetService('TeleportService'):Teleport(814847290, game.Players.LocalPlayer, {}, transitionFrame.Parent)

I want the game to not appear breifly when loading into a new place and when loading into the start place. I have no idea really how to fix it and when I read the wiki I saw this message

There is a small delay when waiting for the PlayerGui. If you hide the default loading screen before you wait for the PlayerGui to load the game will be visible for a brief moment. It is recommended to wait for the PlayerGui to load first, insert the initial screen you want to display, and then hide the default loading GUI.

I don't really understand what this means