Read this interesting article with more details.
Simple explanation :
An atomic transaction is an indivisible and irreducible series of database operations such that either all occur, or nothing occurs. A guarantee of atomicity prevents updates to the database occurring only partially, which can cause greater problems than rejecting the whole series outright.
In cryptocurrency this translates to a trustless exchange. Komodo's BarterDEX and HyperDEX dApps use an atomic swap protocol that is blockchain agnostic and allows traders to swap different coins (e.g. convert Bitcoin to Komodo) without exposing your private keys to a third party. This is known as a "peer-to-peer", or "decentralised" exchange.
NOTE: Komodo atomic swaps are "on-chain", as opposed to Lightning Network (LN) "off-chain" atomic swap.