In computing alphanumeric keys are simply keys on your keyboard that consist of all letters and numbers and some different symbols. These keys include all letters starting from A leading to Z and numbers from 0 to 9.
Alphanumeric keyboards which are the de-facto standard for all computer keyboards are separated into five rows with the top been numeric followed by another row for alphabetical characters. In addition the keyboard contains the middle row for alphanumeric characters, the space bar and all other keys such as control, alt and the function (Fn) key.
Use in passwords
When it comes to choosing secure passwords you should make use of alphanumeric characters such as letters (both capital & lower case), numbers and special characters/symbols such as %$&!#$@.
This is critical to ensure your passwords are hard to crack by automated tools used by hackers, despite been harder to crack you should avoid using simple words or phrases which could be easily acquired by hackers.
Check out this article on choosing secure passwords which will help you generate random secure passwords for your online accounts.