It's an invitation to eat in a restaurant or somewhere outside.
defines "" as follows:
> 1.
>
> 2.
>
>
So can mean a nice meal whether eating at home or outside. But when we invite someone over for a meal, normally we wouldn't use the expression for a meal we serve. We would say or something like that.
So someone says "", he/she wants to treat you to a meal outside.
But when someone invites you to his/her house for a meal, you CAN say "".