Alphanumeric phrases consist of both letters and numerals, or characters.
Alphanumeric means that something is consisting of both letters and numerals, or a character that has either a letter or a number.
What the use of alphanumeric phrases means in crypto is that the more you mix, match and combine characters, letters and numbers, the lesser the chances are that your account will end up becoming compromised.
A primer example of this would be something that includes any single one of the letters from the alphabet, as well as any number from 0 to 9. A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, and Z are all alphanumeric characters, as well as 1,2,3,4,5,6,7,8 and 9.
These are typically used in passwords in order for them to be hard to discover or hack. Examples of passwords such as these include nfgh1zya6buh1656, 22s7sq4wysltjiaq, g93vvtmlslz4htfs, 6dxa7a3ibiajifzc and many others.
You will find that almost every single cryptocurrency exchange out there will require you to create a strong password in order to use the exchange itself; this is mainly due to the fact that they want to protect you as much as possible.
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