Alphanumeric refers to something that has both letters and numerals, or a character that is either a letter or a number.
In the realm of cryptocurrencies, this is generally associated with creating a password for your crypto wallet or exchange, where you must ensure that you use both letters and digits to obtain the highest level of account security. This essentially implies that the more you mix, match, and combine things, the less likely it is that your account will be hacked.
A simple example might be something that has all of the letters of the alphabet as well as any number from 0 to 9. These are commonly used in passwords to make them difficult to find or hack. Passwords like these include ykugwrjk61656, 98fhrf8hj761sltjiaq, jgrf8h5647hbdf, 7hfgfsb7665vdxhd9z, etc.
Almost every cryptocurrency exchange will need you to establish a strong password to use the exchange; this is mostly due to the fact that they want to protect you as much as possible.