Metaprogramming refers to manipulating or extending a program from within itself. ES2015 introduced the Proxy API for metaprogramming in JavaScript. A Proxy allows defining custom behavior for fundamental operations like property lookup, assignment, and enumeration. It works by wrapping a target object and defining traps (functions) for operations. While proxies enable powerful abstractions, they can impact performance due to the added layer of indirection. However, use cases like validation, logging, and virtual objects demonstrate how proxies can extend functionality transparently.