I'm not sure if this is the root.json that you're describing, since you haven't listed any identifying contents or folder paths in your question, like this:
But if this is what you're looking at, then it looks like this file describes the start of one of the "Advancement" trees in the game. For this example, it's the first achievement in the "Minecraft" / "Story" tab (on the far left of this image):
The first part of the file covers what should be shown for this advancement in the UI, including an icon image (for the first/root node in the tree, this icon is also used for the menu tab itself), localized text IDs for its title and description, and what icon frame shape the advancement should use.
It also defines whether the advancement should show an on-screen toast or be announced to the chat upon completion.
The root (first) node in the advancement tree also defines what background texture to use for the menu tab containing its tree.
The next section defines the trigger conditions that will lead to the advancement completing - in this example, when the player gets a crafting table in their inventory:
Other json files in the folder look quite similar, but they'll also have an extra line at the top:
...which indicates which node comes before them in the tree.