Re your first question - Although your tutors notes are not explained too well - I agree that the Table Name should be Designed and saved as "House Lettings"
On the Second part re Validation - A Relational Database Management System allows you define the kind of data you have and how the data should be stored. You can also define rules that the RDBMS can use to ensure the integrity of the data being input - Validation
So, your Validation Rule for the FuelType Field should read :-
"O" or "G" or "S"
And the Validation Text :-
O or G or S
If the inputter enters a value in the FuelType that does NOT conform to O or G or S then the Validation Text part kicks in and advises you that it this input is not what it is expecting and will not allow you to proceed until it conforms to your Validation Rule.