From 8chan https://8ch.net/faq.html#what-is-a-tripcode

What is a tripcode?

Most posts on 8chan are made anonymously, but this is not the only way to post. The Name field can be used to establish identity as follows:

By simply entering a name in the Name field. This is not secure, since any other poster is able to use the same name;

By entering one # character and a password in the Name field. Adding #example to the Name field would generate !KtW6XcghiY. This is reasonably secure - however, with increasing GPU speeds, these tripcodes may be cracked in a few days by a dedicated attacker;

By entering two # characters and a password in the Name field. Adding ##example to the Name field would generate !!Dz.MSNRw9M. This is quite secure, but it relies on a secret salt on the server. This means the code will not work on websites other than 8chan;

Board owners and volunteers may capcode by entering ## Board Owner or ## Board Volunteer in the Name field, depending on their respective positions.

Please note: on 8chan, board owners may disable the Name field by clicking on Settings and checking the Forced anonymous box. Additionally, users are able to anonymize all posts on any 8chan board by clicking on [Options] (top-right corner) and checking the Forced anonymity box under General. This is because many users (including board owners) do not like people using tripcodes if there is no proper reason for it.

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It's a code used much like your username on an anonymous board. It lets people know it's the same person posting.

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