It can be a controversial subject, especially when funding for special education services is an issue. Everyone's profile of abilities includes strengths and weaknesses. Some people's weaknesses cause a great deal of interference in learning. The interference may come in reading, math, writing, or other learning activities. Some learning weaknesses may cause difficulties in understanding social situations.
Psychological testing can show the child's profile of strengths and weaknesses.
This is a surprisingly complex question, and has been the subject of federal and state laws and the cause of extensive bureaucratic procedures within local school systems. It has also been the subject of considerable scientific research. We might not immediately think of inability to read, but this can impair a person's life severely. Sometimes children with learning weaknesses are not identified, because they compensate so well for the things they have trouble doing. People naturally try to use their strengths to compensate or make up for the interference caused by weakness.