frequency... how often, in a population of individuals, will a particular allele occur.

In very basic terms - an allele is a specific form of a gene that brings about ( or at least can bring about) a trait in the individual - though you may also have alleles that do nothing other than exist so there is nothing to select for or against. The most common examples are for eye color or blood type... and it is mutations that alter alleles that provide variations in gene types.

If an allele provides a trait that is advantageous... either because it helps an individual to survive or win a mate & produce more offspring, then that allele has the opportunity to increase in frequency in a population.   Over time, the makeup of the population will change to reflect the selection of alleles in the population. Given enough time, the individuals in that population will differ from their earlier ancestors enough to regarded as a "new" species or subspecies. Much like history... it reflects the changes over time.