If you look in the top right corner of the left image, and the top left corner of the right image, you'll see the words "Select" and "Candidate". The "Select" image is the one that will be processed if you use a shortcut to go to develop mode (e.g.; 'D' or 'R').
The "Make Select" icon replaces the current "Select" image with the "Candidate" image. So if you're looking at the two, and you like the one on the right better, you click this icon to eliminate the one you like less. If you've selected more than 2 images to compare, the next image slots into the "Candidate" position. Lather, rinse, repeat until you've gotten down to the 1 "best" image.
Up and down arrows? I have left and right arrows, but not up and down. (see attachment)
The right arrow replaces the "Candidate" image with the next image in your selection. So if you're comparing 2, and prefer the current "Select" image, this gives you a new "Candidate" to compare with the "Select." The left arrow is the reverse of this, going back to the last candidate image.
I think if you experiment a little, it will become obvious. In the grid view, select 5 or 6 images that are easy to tell apart, and press "C" to go into compare view. Now press each of the icons and watch what happens.
