ABA stands for American Banker's Association, a banking industry association that developed the routing transit number system used for paper checks and electronic funds transfers sent via Automatic Clearing House (ACH).
Routing Number is a 9-digit identification number commonly found at the bottom of a check, used by financial institutions to identify where a bank account is located. In this regard it is similar in nature and function to how a zip code works for residential and commercial addresses.
This sample check image shows where ABA routing numbers can be found on your checks. (Refer to your own check for your specific numbers.)