<layout id="normal" minimum_h="116" maximum_h="116" minimum_w="275" maximum_w="275" droptarget="pldr">
Most of that is pretty self-explanatory, it controls the min and max height and width. The droptarget tells Winamp to play any files you drag and drop on the main window.
Chances are you won't want the rest of this as most skins don't use the standard frame for their main window. But, in case you want to...
x="0" y="0" w="0" h="0" relatw="1" relath="1"
This tells Winamp which layout of the standard frame to use, in this case NoStatus, which means it won't put a status bar at the bottom of the window(like the ADD, DEL, etc in the playlist editor) The x, y, etc tell Winamp where to put the contents in frame, and how big to make it. Content tells it what to put in the layout(in this case whatever you placed in your player.content.group). The padtitle lines are used to for controling the spacing of the title and streaks. The shade line tells Winamp which layout to switch to when the mousetrap is double-clicked. The mousetrap is just an invisible rectangle that catches clicks.
<sendparams target="window.titlebar.title" default="WINAMP"/>
This determines what name will show up as the title. Set default to what you want the name to be.
<sendparams target="mousetrap" dblClickAction="SWITCH;shade"/>
This tells mousetrap what to do when double-clicked. SWITCH is the action, shade is the param.
<sendparams target="window.top.left;window.top;window.top.right;window.left;window.right;window.bottom.left.top;window.bottom.left;window.bottom.left.bottom;window.bottom.top;window.bottom;window.bottom.bottom;window.bottom.right.top;window.bottom.right;window.b ottom.right.bottom;" resize="0"/>
This tells winamp not to allow resizing with the pieces of the standard frame since the main window isn't resizable (in this case.)
<sendparams target="window.bottom.right.top;window.bottom.right;window.bottom.right.bottom;" scale="bottomright"/>
This tells Winamp to allow the bottom right standard frame parts to rescale the window.
This hides the close and close inactive buttons defined in the standard frame since they defined their own outside of the frame(the ones you pasted before.)
As I said, in most cases you won't use this, but there it is. Let me know if you need any clarification or have any other questions,