I make that mistake and im so tired of it, please explain this to me...
I am not a expert at scripting, I have only been scripting(successfully) for about three months, but since no one has answered you, I will give it a go. PlayerGui is what is currently on that players screen. If it is in StarterGui, then every time they join or respawn, it will show up on their screen. For example, If i have a ShopGui that opens when i click a block, I would have the Gui's Parent be PlayerGui, but if I had a Health bar Gui, then i would put it in StarterGui, so that everytime I respawn, the health bar is there.
make sense? If so please give a thumbs up! If not, I am so sorry!