Percentile data must be enabled on a per measure basis if it is to be calculated, it's not done by default. So that might be one reason why you're not seeing values.
Also, percentile data is calculated hourly, so you will have to look at timeframes of minimum 1 hour to see any values. That might be another reason for not seeing data.
Generally, percentiles are a statistical measure indicating the value below which a given percentage of values fall relative to all the results in that set. For example: if the 20th Percentile for the PurePath Response Time measure is 600 ms that tells me that 20% of the PurePaths in that timeframe have Response Times below 600 ms.
Hope this helps.
It should be mentioned that most agent-based measures have their percentile calculated OOTB. Typically users will user the top percentiles in order to address issues affecting a small % of customers but that does affect them. If the average response time is 1s and that's within SLA that's fine. But if we have 100k customers every hour and the 99th percentile is showing a large deviation from normal... we're still affecting 1k customers. This is why smart baslining gives performance for the top 10% of transactions.