Immediately go to your settings and Disconnect the App you used to generate that token (e.g. Twitch Chat OAuth Password Generator), this will invalidate the token. Re-requesting the token used to invalidate previous ones generated with the same app/account, but I think this may not be the case anymore, so you have to revoke access from your Twitch settings as mentioned to be safe. You can then generate a new token for your actual use.
The access token is sort of a revokable password associated with your account and specific types of access, so sharing the token publicly means everyone can do whatever access is associated with it under your name (in this case log into chat).