Can someone ELI5 to me for Bitcoin Atomic Swaps?
I think this is easier to understand if you abstract a little bit. On a high level, an atomic swap is very simple: the two parties set up transactions such that they can both take their coins be revealing the preimage to some (pre-decided) hash H. Initially one party knows the preimage. They must reveal it to take their coins, so the act of taking their coins is sufficient to enable the other party to take their coins.

More specifically the Tier Nolan in the OP works as follows.


A chooses a random x and gives H(x) to B.
A sends her coins to a 2-of-2 output that can be taken by B if he reveals x. Similarly B sends his coins to a 2-of-2 output that can be taken by A if she reveals x. These are TX1 and TX3, respectively.
(Before broadcasting these, they create locktimed "refund" transactions TX2 and TX4. It's important that A's locktime is much longer than B's, so that if she gets her refund, B has had a long time to take his refund, and is not at risk of A revealing x to also take his coins.)
At this point, A takes her coins, revealing x.
Seeing this, B learns x and takes his coins.


There is a variant of this called coinswap where x does not hit the blockchain, where steps (4) and (5) are replaced by A revealing x off-chain and then the signers just give each other the coins, only using x as a fallback. This complicates the locktime structure and also requires differently-shaped transactions to keep x and its hash off the blockchain, which unfortunately make the protocol look super scary, but that's all that's going on.