WebAssembly runtimes are the engines that execute Wasm bytecode, enabling near-native performance for web and server applications. They provide a secure sandbox where code from languages like Rust or C++ runs without direct system access. This allows developers to port existing libraries and build complex tools like video editors directly in the browser.

The primary advantage is performance, making computationally heavy tasks feasible on the client side. Security is another key benefit, as modules are isolated and cannot access sensitive resources without explicit permission. Portability across browsers, servers, and edge devices simplifies deployment, though debugging can be trickier than with traditional JavaScript.

Popular standalone runtimes like Wasmtime and Wasmer extend Wasm beyond the browser, supporting the WebAssembly System Interface (WASI) for system interactions. As the ecosystem matures with better tooling and features like garbage collection, Wasm runtimes are becoming essential for building efficient, secure, and portable applications.