What is this?

basically the context of the function

it depends on how the function is called (there are many rules that affects the context of this)

1. called with new? use newly constructed object
2. call() and apply() and bind()? use specified object
3. called with context object owning the call? use context object
4. default: undefined in strict mode; else global object

more detailed rules of this here (opens in a new tab)