Snomasks are server notice masks. It's a special type of user mode that controls which server notices you will receive. Only IRCOps can set snomasks.
It can be set by: /MODE yournick +s SNOMASK, for example: /MODE yournick +s +cF. To remove certain snomasks, use something like: /MODE yournick +s -c, or to remove all snomasks use /MODE yournick -s.
The current available snomasks are:
|b||blacklist||Messages from the DNS blacklist module|
|D||snomasks/dccreject||Rejected DCC's due to Deny dcc blocks|
|F||built-in||Far connects (from other servers, except from U-lines like Services)|
|e||built-in||'eyes' notices - a bit verbose (?)|
|j||built-in||Junk notices (noisy, possibly harmless things, mostly for debugging)|
|G||built-in||GLINE and SHUN usage|
|n||built-in||Local nick changes|
|N||built-in||Remote nick change notices|
|q||built-in||Deny nick (QLINE) rejection notices|
|s||built-in||Receives server notices (includes the most important messages)|
|o||built-in||Oper-up notices (/OPER usage)|