It's very unspecific. It's like saying someone has "an infection", which could mean a common cold or always fatal rabies.
Most are genetically inherited conditions, in which the body fails to produce a necessary enzime to metabolize a given substance, which will then accumulate and cause harm. The blood test done in newborn kids has the purpose of screening for a few of these, as if you know which they are you can then change the diet of the child to avoid the nutrient that accumulates before it causes irreparable harm.
They're hundreds or thousands of different diseases though, some always fatal in childhood, some that progress slowly throughout life and cause increasing problems and some that don't affect life expectancy at all and are a mere biochemical curiosity.
It may as well refer to common coditions such as diabetes or high cholesterol.
No idea what they mean here in particular, and the implications are a guesswork.