What is the problem with inbreeding

It reduces genetic diversity and increases the proportion of deleterious recessive alleles

Explanation:

Inbreeding occurs when two genetically-related organisms mate and produce offspring. The breeding of genetically close individuals results in a loss of genetic variation in the population, as there is less "mixing" of different alleles to produce heterozygous (possess 2 different alleles) individuals.

This means that there is an increased chance that any deleterious alleles will become expressed (as there is less variation and more likely that homozygous individuals exist). These can be deleterious and potentially disease-causing. This reduces the fitness of the population.