There are two different potential causes of inbreeding in a population:
Breeders intentionally mate sires and dams that are more related to each other than the population average (also called non-random mating). This results in a temporary loss of genetic diversity and is called avoidable inbreeding. From a population perspective, avoidable inbreeding due to non-random mating is not an issue. After one generation of random mating these effects will have disappeared.
Breeders cannot avoid mating sires and dams that are related because all animals in the population are related to each other. This results in a permanent loss of genetic diversity and is called unavoidable inbreeding. The unavoidable inbreeding cannot be reversed using a certain mating strategy.
The size of the breeding population, and the relative size of the contributions of each of the breeding animals, will determine the rate of unavoidable inbreeding. In each generation further in the pedigree the number of ancestors increases exponentially. E.g. in generation 10 an animal has 210 = 1024 ancestors. In most breeds in the period the ancestors of generation 10 lived, less than 1024 animals were used for breeding. Thus further in de pedigree the same animals pop up in the pedigree of the sire and the dam: they are related and therefore their offspring becomes inbred. This emphasizes that the deeper the pedigree, the better the relationship between sire and dam can be established.