what is the principle behind Allele Frequency Net Database
Allele frequency is simply the frequency at which a specific allele is found in members of a population. Let's take the following example from the site you referenced:



The first column merely has the name of the allele, A*01. The second column is the population from which individuals were genotyped. Skipping ahead, the fifth column is the number of people genotyped from the population, 187 in this instance. Of those 187 people, 14.8% of them have at least one copy of the allele as stated in the third column. It's important to remember that each individual could potentially have two copies of the allele. 

The fourth column gives the actual allele frequency, calculated by the number of A*01 alleles/number of total alleles (374, assuming no serious mutations). Of those 374 total alleles, 7.4% (0.074) of them are the A*01 allele, meaning 27-28 of the 187 total people had the allele. Since 14.8% of people had the allele, but the frequency of the allele was only 7.4%, we know that each person who has the allele only has one copy of it.