Those messages 99% of the time come from Services called 'NickServ' and 'ChanServ'. As Cobra pointed out, they are for that specific network/server, and not related to mIRC, mIRC is simply a client which connects you to the IRC network. Not all IRC networks have Services - others that do may not have NickServ/ChanServ, but "service bots", usually represented by a single letter, such as 'Q', 'L' or 'X'.
For NickServ/ChanServ help you can almost always use /msg nickserv help
or /msg chanserv help
. Some networks have disabled this for security purposes, so /nickserv help
and /chanserv help
(Note that there is no /msg) may only work.
The best place to obtain help with Services for your network is to join the #Help channel with /join #Help, and ask there. Alternatively, check out the network's website - you could ask #Help what the website address is if you don't know it, or search for it with a search engine such as Google