The black line represents a branching path you can take in a game.

There are points in a game that are impossible to return to after leaving the area or like killing a certain boss enemy. (Red X in the image)

When there are cool/special items only accessible in those areas (yellow star), if you leave and can’t come back, you might miss the item and be unable to get it ever without restarting and playing the game all over again just to get the chance to pick it up this time around.

Things get much worse when there are multiple items or even boss fights like this in the same game, where doing things ouf of order can lock you out of full questlines (like in Elden Ring where reaching Altus Plateau before getting as far as possible through everyone’s questlines will make half of them unable to be completed for no reason whatsoever)

I’m about half that age, and I’ve met at least a couple grads and undergrad students on here that I assumed were also in their 20s…