It is the environment where JS code is evaluated and executed. It has two phases: memory allocation (variables get undefined, functions are stored) and code execution (values are assigned, functions run).

A global execution context is created once when the program starts. A function execution context is created every time a function is invoked, pushed onto the call stack, and destroyed when the function returns.

Hoisting happens in the memory allocation phase. Variables declared with var get undefined, function declarations are stored in full. This makes them usable before their line of declaration.