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It means that in some place of the application any type of symbol is accepted - letters, numbers, spaces, special symbols.
A from z
Composed of letters and numbers
Alphanumeric characters means any combination of letters and digits. Sometimes other special symbols are allowed, too - so alphanumeric may sometimes be understood to mean a combination of just about any type of symbols.
Decide what symbols you want to allow. For example: 26 uppercase letters + 26 lowercase letters + 10 digits = 62. Or less, if you don't want to distinguish uppercase and lowercase; or more, if you want to allow certain special symbols such as spaces, hyphens, etc. Raise this number to the sixth power, if you use 6 characters.