This is called LINQ(Language Integrated Queries), with the help of which which at the back end this gets converted to normal Sql queries which you can also check using a very nice tool called
LINQPAD[
^]
Here in this statement you have mentioned, this query
returns a list of integers from the Table named tblAdvert with the help of its context i.e. tblAdverts(generally context names are pluralized), where a filter is added based on the BusinessUserId and Grouped by plans column and then select the key.
This iswhat in general this linq means.
But I am unaware of why you have used key.Value here.
I hope I could make you understand somehow.
Post back your queries if any.
Thanks.