Explain Like I'm Five

Kids today can't even read the Song of Pelinal Whitestrake smh

“The thing people don't realize about the gear wars, is that it was never actually about the gears at all…”

Story and lore are two very different things

The main story in Skyrim is that you play as Dovahkiin, Dragonborn, in the year 201 of the 4th Era. After having been unseen for centuries, dragons have returned during a time of civil war.

Oblivion takes place about 200 years earlier, 4E433, in the province of Cyrodiil to the south of Skyrim. It takes place after the assassination of the Emperor and during the Oblivion crisis, during which portals to Oblivion open allowing the Dadric prince Mehrunes Dagon’s Daedra to walk the land.

Morrowind takes place only 6 years before Oblivion, to province of Morrowind lies east of Skyrim and northeast of Cyrodiil. The game takes place during a time where the empire has been actively colonizing the isle of Vvardenfell, the events largely revolve around the mad-god Dagoth Ur.

Each game’s story is more or less self contained, you don’t need to play one in order to understand and enjoy the next.

For games that chronologically take place after the others, you will find references to the events of those games in the world as lore.

Which brings us to lore - nobody can really ELI5 or “short story” lore. Lore is all of the bits and pieces that make up backdrop for the entire game world, it comprises the contents of all of the books in the game, stories and anecdotes shared by NPCs, etc.

Far as whether or not you give the story and lore your attention - completely up to you and how immersed you want to be. You can mostly cruise through Oblivion and Skyrim as action games with little regard for the story. Morrowind is a lot less approachable in this regard, since it requires a lot of reading in order to progress the quests and story. It’s still quite fun as an open-world sandbox, which is how I treated it when I first played as a kid.

No you don't need to go back. IMO the main quest, or at least the way it is delivered is not great anyway. It's mostly a way to send you around the map to naturally bump into other quests and activities. To ELI5 the story:

Both have very little over arching story. Most of it is flavor stuff for the immediate area, or the next area in the quest. If you ever played cyberpunk 2077 it's a lot like that, where you get a little insight on the person you're about to murder.