This way of communication between processes very much resemble to the communication we do with our mail accounts. I would guess (smartly, no doubt) that was the key factor used for naming the functions when they were created.
Information about mailslots can be found on microsoft's site, link. Summary could be that datagrams are used for communication and it's one-way and asynchronous.
Attached zip file contains MailSlot.au3, AutoIt's implementation of these functions.
Also there would be two scripts in there MailSlot_Sender.au3 and MailSlot_Reciever.au3. Both scripts are demos of the mailslot mechanism. Start both and use former to send mails to latter.
Examples are basic and should show what's this about.
Available functions are alphabetically:
New attachment. _MailSlotGetTimeout() function added to MailSlot.au3.