A guild charter is like an info page for a guild, often used to give additional information about the expectations a guild has for their members, the games they play, guild rules, etc.
Taken from Guild FAQ:
https://forums.d2jsp.org/info.php?p=5Quote
Q: What is a public Guild Charter?
A: A public Guild Charter is a brief description about what your guild is, and what it stands for. Typically, you outline your rules and guidelines, and any guild policies and/or requirements you may have for the members. This is a publicly displayed message, that basically informs new potential members on what is expected of them to become a member.