Using the IN clause with a WHERE is a simple way to tidy up a query, making it much more readable, and potentially increasing query performance.
SELECT * FROM Person WHERE PersonId IN (1, 2, 3)
The above is equivalent to
SELECT * FROM PERSON WHERE PersonId = 1 OR PersonId = 2 OR PersonId = 3
As you can see, the second query contains a lot more syntax for the same result.
You can also use a Noncorrelated Subquery within the brackets of the IN clause, as below:
SELECT * FROM Person WHERE PersonId IN ( SELECT DISTINCT PersonId FROM Orders )
The above code would return all Person records, who have a record in the Orders table.
Alternatively, you can use NOT IN to exclude specific records.
SELECT * FROM Person WHERE PersonId NOT IN (1, 2, 3)
This would return all records that do not have a PersonId of 1, 2, or 3.