A percentile is the share of the comparison sample who scored below you. The 72nd percentile for extraversion means that 72 out of every 100 people in the sample scored lower than you did, 28 scored higher, and nothing more than that.
Why the site uses percentiles rather than raw scores
A raw score of 38 out of 50 means nothing on its own, because it depends on how the items are worded and how people in general answer them. On most trait scales people cluster towards the middle and a little above it, so a score that looks high out of fifty may be ordinary. The percentile puts the raw score against real people. The strip on every results page draws that comparison: the curve is the share of the sample at each raw score, the darker middle holds the half of all people between the 25th and 75th percentiles, and your marker sits at your raw score with your percentile beside it.
How it is calculated
The site holds, for each scale, a count of how many people in the sample scored each possible raw score. Your percentile rank is the share scoring below you plus half the share scoring exactly the same, times a hundred. That mid-rank rule means a score in the middle of a symmetric sample is the 50th, and two people with the same raw score always get the same percentile. The method and every sample size are on the methodology page.
What the sample is, and is not
Every comparison sample on this site is a group of people who chose to take the same test online, released by the test’s author or by a research project. They are large (hundreds of thousands of people for the Big Five tests) but they are not representative of any country: they are younger, more female and more English-speaking than the population. A percentile tells you where you fall among people like the ones who took the test, which is the honest comparison available, not where you fall among everyone. For the interest profiler, the developers publish percentile ranks only for a US developmental sample of about a thousand people, so the site shows counts of ticks rather than presenting those as your norms.
What it means for your results
A percentile is a comparison, not a judgement. The 95th percentile is not better than the 5th; it is rarer. Half of everyone is between the 25th and the 75th, so a result in that band means you are like most people on that trait, which is the commonest and least remarkable result there is. Small differences in raw score can move the percentile a lot near the middle of the curve, where people are crowded, and hardly at all near the ends, so do not read anything into a shift of a few points between attempts.