this

The this keyword provides a way for the object instance that is currently executing code to refer to itself.

this in Unity

In Unity, most custom code that we write is to create script components. Therefore, in Unity, this usually refers to the script component that is executing the current code. Common uses include accessing a different component, or identifying the associated gameObject.

Vector3 pos = this.transform; //example of accessing a component

Destroy( this.gameObject);  //example of identifying associated gameObject

MSDN Reference

The this keyword refers to the current instance of the class ...

The MSDN reference provides several usage examples