Percentile 90 or P90 is the value below which 90% of the data fall. For example, let's say you have 1000 User clicks, and a P90 value of 8 seconds, then you would know that 900 of those clicks would be shorter than 8 seconds and the remaining 100 (10%) would be longer than 8 seconds.

Same logic applies to p95, p99, p999