When you declare a variable as being Dynamic, you will be able to assign any value to it at compile time. So you can actually compile this code:

class DynamicTest
{
public static function main()
{
var dynamicVar : Dynamic;
dynamicVar = "Hello";
dynamicVar = 123;
dynamicVar = {name:"John", lastName : "Doe"};
dynamicVar = new Array<String>();
}
}

The compiler won't mind even though you are assigning values with different types to the same variable!