95th percentile simply means that every 5 minutes a sample of bandwidth usage is taken. At the end of the month the top 5% is tossed and the next reading is your month's usage.
Now say you spike 10Mbits for 36 hours of the month... and then the other hours of the month are 1Mbits, you're only paying for 1Mbit at the end of the month. If you spike for more than 36 hours... the highest spike after the 36 top hours, is your monthly Mbit usage..
Hope you understood that. It's rapidly becoming a standard is usage measurement on dedicated servers. You can read more about it here:
http://www.sitespecific.com/about/95th_percent.asp