There are different subtypes of this disorder that can be pretty easily understood in non-technical language. But speaking is not like toilet training, it just isn't true that everbody gets there. Further, what you do as a parent can have an enormous influence on your child's eventual language skills. Speaking to them, reading to them - those activities are key, but are not enough for some children. For those children, early intervention is the key to success.
Early intervention is never harmful and often critical - depending on the child.
These first 5 years of life represent very special years in terms of the ability for the maturing human brain to incorporate language. It is a critical period . After that, if language as a concept is not learned it is likely impossible to attain it. During this critical period, language skills are exploding and the earlier a barrier in learning language is recognized and removed, the better. There is a range of normal ages for expression of sounds just as there is a range of normal ages for sitting up and walking.