EVM-compatible means a blockchain can run code designed for the Ethereum Virtual Machine, so standard smart contracts and many developer tools work with little or no change. In practice, networks such as Polygon can execute familiar contract logic, read contract state, and process transactions in a way that resembles Ethereum.
This matters in cyber security because compatibility makes code portable across multiple chains, which helps legitimate applications but also helps attackers reuse infrastructure and techniques. Malicious operators can place command data in smart contracts, move control logic onto a public ledger, or switch between EVM-compatible networks to reduce dependence on a single server. Defenders can use the same compatibility to their advantage by tracking contract addresses, wallet activity, RPC traffic, and unusual endpoint requests to blockchain nodes or gateways.