Snomasks are server notice masks. It's a special type of user mode that controls which server notices you will receive. It is mostly used by IRC Operators.
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||IRCOp-only|
|D||snomasks/dccreject||Rejected DCC's due to Deny dcc blocks||IRCOp-only|
|F||built-in||Far connects (from other servers, except from U-lines like Services)||IRCOp-only|
|e||built-in||'eyes' notices - a bit verbose (?)||IRCOp-only|
|j||built-in||Junk notices (noisy, possibly harmless things, mostly for debugging)||IRCOp-only|
|G||built-in||GLINE and SHUN usage||IRCOp-only|
|n||built-in||Local nick changes||IRCOp-only|
|N||built-in||Remote nick change notices||IRCOp-only|
|q||built-in||Deny nick (QLINE) rejection notices||IRCOp-only|
|s||built-in||Receives server notices (includes the most important messages)||IRCOp-only|
|o||built-in||Oper-up notices (/OPER usage)||IRCOp-only|
You can control which snomasks you automatically get (set::snomask-on-connect) and which you get on oper (set::snomask-on-oper, oper::snomask)
If a user simply sets mode +s then certain snomasks are set by default, see also set::snomask-on-oper.