The human genome is made up of genes, which control the production of proteins for different functions, like the colour of your skin, the size of your heart etc
These genes vary from person to person, but some genes are defective. For example there might be two versions of a gene [called alleles]
Let's use flowers as an example:
One allele might code for purple pigments to attract bugs, another one for white flowers [a defective version].
Because you have two parents you have two sets of alleles for most things, same with the plant. So if the
planet has 1 purple allele and 1 white allele it can still attract bugs because 1 allele is producing the right pigments despite the defective gene [this is called a carrier]. But if you have two sets of defective flower genes your flower won't be purple and the plant won't be able to attract bugs to pollinate it. [this is called a sufferer] :C
So how does this relate to incest? Just like the plant you
don't want to be born with two sets of defective genes [although for you it could be something like a heart disease rather than a crappy flower] In a family a brother and a sister might both be carriers for this gene, so if they have sex with eachother the chance of the offspring having two sets of defective genes [a sufferer] is much higher than if the siblings had sex with people from a different gene pool who were not carriers.
/end obligitary and crap explanation/