A routing number is a 9-digit code that identifies your specific bank or credit union. It's the same for every account at that institution's branch network — it doesn't change from check to check or account to account the way your account number does.
Where to find it
It's the first group of numbers printed in the MICR line at the bottom of a check, and it's also usually available in your online banking portal or on a voided check.
What it's used for
Routing numbers are used to direct payments — checks, ACH transfers, direct deposits, and wires — to the correct bank before the account number tells the bank which specific account to credit or debit.