Sure thing, mate. In super simple terms, the "longest chain" in Bitcoin is the version of history that has had the most work put into it.

When miners solve complex mathematical problems to add a new block of transactions to the blockchain, they're actually putting in work (in the form of computational power). This process is known as Proof-of-Work.

So when we talk about the longest chain, we're talking about the blockchain version that miners have put most work into. And according to Bitcoin's rules, this is considered as true record of transactions.

It's like if you and your mates were telling a story together piece by piece and each part took some effort to create - The longest story would be one where you guys spent most effort!