SQL permissions based on record value (if column X = Y for user1 allow Inserts/Updates etc.)

I need to run queries as a "user" which is a record in a table, with permissions based on a record value of that user.

I have a database with a tUsers Table, such as:

ID    Username       Email         Role
1      Pieman       mail.com       Admin
2     Cakedude     mail.co.uk   Receptionist
3      Muffin        gh.com        Other

I need to have it so only "users"/records with "Role" of "Admin" can view and edit the table, and "Receptionist" view it etc.

I know of and use GRANT for permissions, but don't know how to run a query as a user based on a table record and to have the permission only GRANTED if that users' role is "Admin"

So if I have:

USE DB1;
GRANT SELECT ON OBJECT::tUsers TO Admins;
GO 
SELECT * FROM tUsers

How do I make that run as say tUser with ID 1, and the GRANT if the users' role = "Admin"

I'm sure I've seen this done before.

I'm fairly new and still learning the correct terminology, so if this is a duplicate question, or is essentially just describing an sql Function sorry.