Unless you are a speech pathologist or a parent who has a child affected by a phonological disorder, you likely have no idea what this is. You may be somewhat familiar with articulation disorders (like lisping S or saying W for R), you surely know a bit about stuttering, but phonological disorders are something that aren't part of what most folks think about when they think of speech disorders.
A phonological disorder exists when children continue to use phonological processes beyond an age-appropriate level. For a time, phonological processes are something young children use to simplify speech and language so that they can learn it more easily; and this is normal. However, as they develop, these processes should be outgrown. When they are not, this is considered to be a phonological disorder.
Some phonological processes that are typical in early speech development may include initial consonant deletion ("at"/hat); consonant cluster reduction ("nay"/snail); reduplication ("wawa"/water); final consonant deletion ("ma"/mat), among many others. There are different ages and stages at which these processes should be eliminated from a child's productions. When this does not happen, it is considered disordered.
When I explain this to parents, I describe it as an umbrella. Unlike an articulation problem, which is sound-specific, phonological disorders, cover possibly many sounds under the rule with which they govern. For example, a rule in American English is that we put ending sounds on words. Imagine an umbrella that has this rule written on it. Under that umbrella may fall T ("ha"/hat); S ("mou"/mouse); or N ("ca"/can). A child may be able to produce those sounds, but they have not incorporated the rule that we include sounds at the ends of words, so in those instances, it will be deleted. The umbrella represents the rule, and any sound may fall under it.
Phonological disorders, although not as well known, can greatly impact intelligibility. Many children who have many phonological processes present are highly unintelligible and need intervention. If you are concerned that your child may have a phonological disorder or is having any other speech, language, or communication difficulties, please contact a speech-language pathologist.