I'm at a disadvantage here because I don't do Mac or MAMP but:
1. The .sql file is what's called a SQL dump file. It is just a text file full of SQL instructions that you can view in a text editor. It works the same as if you typed them in by hand.
2. You can run those instructions to create the database for you by importing or "run a batch file" depending on what program you are using (Not that you would want to).
Have a look here:
Exporting and Importing a database using phpMyAdmin
phpMyAdmin comes pre-installed with most hosts.