Messaging is APRS's version of texting for hams.

Message ACKs

Messages sent to a station are transmitted with a message id. When the message is received an "ACK" is sent back referencing that message ID, so you know it got there. Messages are repeatedly transmitted until an ACK is received, the interval between retransmissions increasing after each try up until a time out.

Reply-ACKs

Sometimes ACKs just don't make it back through the APRS network for some reason or another, and messages would tend to show up as undelivered even if they were. To address that, when you *reply* to a message you've received, the message ID also shows that it is a reply to another (by referencing the ID of the message to which you're replying). Receipt of the reply message then serves as notice that the original one was received. These are called "Reply-ACKs".

Send Message To: Open a dialog that allows you to send a message to a particular station. If Xastir is able to determine what type of station you're corresponding with, it might tweak this dialog so that your text will be displayed in the optimum format for the recipient. After you send the message, the dialog box stays open showing you retransmission attempts and whether the message was received (i.e. we got an ACK back). You can "kick" the timer to make it start the decaying algorithm back at the fastest rate, but this should be used sparingly.
Show Pending Messages: gives you a dialog box with the list of all outgoing messages that haven't been ACKed yet.
Open Group Messages: A feature that has never been fully implemented, is intended to provide a sort of "group text" capability. But it was never finished.
Clear All Outgoing Messages: Stop retransmitting any messages that were pending and haven't been ACKed yet.
General Stations, IGate Stations, and WX Stations Query: sends out a general query packet that asks all stations of the specified type to announce themselves. Few modern APRS stations will respond, as this was considered a fairly bad feature of APRS that is abusive to the local APRS channel. You might as well not bother using these.
Modify Auto Reply Message/Enable Auto Reply Message: You can set Xastir up to send back a stock reply to any message you receive. Most don't, and it's probably not helpful to do so in most cases. Folks will know you're not sitting at your station when they get no response, and no response uses less of your local APRS channel's bandwidth than an autoresponse.
Satellite Ack Mode: don't send ACK packets for received messages, to prevent unnecessary transmissions. This is specifically intended for users who are operating APRS through digipeaters on satellites, a fairly niche use. You should probably never enable it unless you're doing this type of operating.