In simple terms, it shows the percentage of all students that scored less than your student.
A Rank of 95 means that your student scored better than 95% of all the students (you could also say that they are in the top 5%)
A Rank of 50 means ‘average’
This is why we say to aim for a Rank of 85 or above. This would put your child in the top 15% which is roughly the percentage that make it through to grammar schools.