(a) chances of promotion
(b) starting salary
(c) medical and dental plan
(d) length of annual leave
If you were asked to put these in order of the importance you would rate each item when considering a job, then it's easy to say that one was 'top of your list', one rated 'third' on your list etc.
Say you have 15 items. Rather than being so specific as to say, "Having a male boss rather than a female boss" rated 14th on the list, it's easier to see items in terms of those that were most important (towards the top of the list) and those of least importance in influencing your decision: those towards the bottom of the list.
In fact, we could regard the list as being in three parts, the top third of the 15 items (1- 5), the middle 5 items (6-10), and the bottom third of the items (11-15).
So - item 13 would be "in the lower third" of the list.