Hi,
  As you've probably noticed, they've only just 
added this feature to their auction. From what I 
have observed so far it appears that the stars 
and the 'hot item' label indicate how much 
interest there is in that individual auction 
item. That is the number of views of that 
particular item. 
  I have personally clicked on a '3 star' item 
and seen it change to a 'Hot Item'. Actual bids 
may also be a factor. 
- X
ps. Please note that this is just conjecture on 
what I have observed so far.