It sends the machine to the home switches. If you don't have home switches installed, than it sets your machine coordinates to zero at their current positions.
Thank you very much for the insight. So my machine doesn't have limit or home switches, so it seems like I should be ignoring the "Ref All Home."
In terms of the RTFM comments from the other users...
For the record, I did try to find this information myself. First, I searched the "Install and config guide" for "ref all home" and it didn't find anything. I didn't check the usage guide because it said 1.84+, which I took to mean it was for an older version, as I'm running 3.043, and I didn't realize the "+" means it is for 1.84 and beyond.
I did a google search for "mach3 ref all home" before posting my question and the first page of hits talked about changing the use of that button.
So after reading these responses I did look at the 1.84+ guide and searched for "ref all home" and still didn't find it. Reading through the manual, it is referred to in the text as "ref all," and the explanation isn't that helpful for a newbie.
So I did try, I just couldn't find it. Sorry for being a painful newbie, I only post when I have failed at finding what I need. Again though, my apologies.