Cross-blockchain exchanging of various coins/tokens without using an exchange or intermediary. Atomic Swap is a technology that allows owners of various coins to exchange these, without having to rely on any third party, making it a trust-less operation, backed by HTLCs (Hash Time Locked Contracts).
The traditional way of exchanging coins would be to take your current coins to an exchange, sell them for bitcoins (for example) and then use those bitcoins to buy the desired coin. With Atomic Swap, you can do it in a peer-to-peer way.
Atomic swap is a new technology and as such, it's still not perfect and requires certain conditions for it to work ... both of the coins to be swapped have to run on a blockchain with the same hash function and they both must have the lightning network implemented.