leave 5% of top values and leave 5% of least values 
remaining all values are 90%tile values

means leave top 5 values of a total of 100 values and leave 
least 5 form them by arranging them in ascending order and 
then take average for remaining 90 values out of 90 only

90%ile means 

Suppose if you take 100 vusers, response time getting by 
the 90th Vuser

90 % of attempts on a transactions uses the response time 
is called 90% ile value.

Ex: 
MIN       MAX     AVG      90%ILE   STD
2SEC      9SEC    5 SEC     8SEC

above metioned min is 2 sec ok. max is 9 sec ok....total 
avg 100 attempts is 5 sec.

here,total avg is 5 sec right. total 100 attempts .....in 
90%ile is avg 8 sec.ok so here total avg  > 90% ile avg. 
that is the bottleneck of transaction reponse time in 
client side metrics, ok. understand.

i will explain u with same above example...

MIN       MAX     AVG      90%ILE   
2SEC      9SEC    5 SEC     8SEC

the above what is your result. Fine.. now answer to my questions, we are running 100 users, 

1)50 users took only 2 secs, so MIN response time is 2 sec
2)1 users took 9 secs, so MAX response time is 9 Secs
3)Remaining users took some where 6 to 7 secs.

Now as per the above results you cant pick the MIN value 2Sec and say this is the response time for 100 users, in the same way you cant pic MAX value 9secs and say this is response time for 100 users, and u cant take avg 5secs also because 50 users means more half of the users took only 2 secs. 

So we need to take the 90% value. Average means 50% value we are not supposed to take it, 90% value will give us a time assuring that this is the most maximum time given to any user who will get response back from the server.

hope you understood this now...