You won't "hear" a thing that's said in the channels. Private messages get through though. Primary reason is making the job easier for service bots that dosn't want to process channel messages.
Invisible people won't show up when searched for with /who, /who #chan and not with /whois with wildcards, unless you can see them already by sharing a channel. To put it briefly, the invisible mode prevents people from finding you unless they know your exact nickname or are on the same channel as you. You can hide by changing nick, people can't mass-message everyone on your ISP and you are not visible from outside a channel. Today more than half of the nicks on Undernet are +i. When not set to +i, a fellow user can locate you through the /who or through the /names #channelname command. IRC Operators also have the ability to see someone regardless of whether they are invisible or not.
You receive various information sent between servers. Mostly kills and G-lines, but also nickname collisions, invalid usernames, splits, etc. Setting yourself +s is a request to the server to send you all the generic server messages and information about oper kills. Undernet has stopped allowing regular users to use +s.
The IRCops can broadcast messages with this. Wallops are network wide messages sent by IRC Operators to each other. Any user who is set to +w will also be able to observe the conversation. Wallops have become quite extinct on undernet and are rarely sent. However, undernet did 'broadcast' the 10th Anniversary celebration on wallops, and plans to use it again for latge events are in the works.
+x is a usermode that allows logged in users to hide their real host, replacing it with @username.users.undernet.org . Log into X ( /msg X@channels.undernet.org login Username Password ) then type //mode $me +x or /mode nick +x . Hiding your hostname will shield you against several types of attacks often carried out on IRC, but be aware that taking part in a DCC chat or file transfer will reveal your IP address. The +x usermode remains as long as you're connected to the IRC server, so that channel service can recognize you even if your server splits off and rejoins the net. No need to login to X all over again.