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.