For example (TSQL syntax may not be correct):
set transaction isolation level serializable
-- If another transaction/statement is doing: update prices set price=99 where product_id=123; Then you cannot read the modified data (price=99) until the other statement outside this transaction commits.
select * from prices where product_id=123;
-- The second statement in this transaction.
select * from products where quantity=1;
-- Another transaction cannot modify (update/delete) the products table where quantity=1, until the end of this three statement transaction. The other transactions can modify rows that are not affected by where quantity=1.
-- The third statement in this transaction.
select * from sales where city='TO';
-- Another transaction outside this transaction cannot insert a new row: insert into sales(city) values ('TO'); till the end of this transaction. They can insert rows other than those that have a value of 'TO'.