If you create a new gallery (or any gallery you have in fact) and click the button to edit the gallery, at the bottom you will see a box to tick that says 'click here to make this your PAD gallery'.
Once you have done this, it means that each time you upload a new photo to the gallery it will appear in the 'meta gallery'
http://www.pbase.com/pad at the time it was uploaded. Only your latest uploaded pic will appear, the previous one you uploaded will disappear from the meta gallery. You only ever have one photo at a time in it.
Most people upload a new pic everyday to this gallery. Or take a pic everyday and upload it when it is convenient. If you don't do one everyday, or put up one not taken that day then nobody will bother too much.
I think the original idea was that if you take a picture everyday, and post it for people to see, then this would improve your skills and technique. And the PAD 'metagallery' came into being so that people could easily see what other people doing the same thing were up to.
But there are no rules and nobody will really mind if you do things your own way.