Accreditation is when a school institution, company, or agency receives an official certificate of quality.
In the context of the law, law schools commonly receive accreditation. Being accredited makes a law school desirable for law students because it means that the school is reputable and is most likely a good place to receive a quality legal education.
Regarding the law school example, there are a number of both regional and national associations that provide accreditation services. These associations will conduct research and inspect the schools to make sure that everything is in order. If a law school does not receive accreditation, then it can have an enormous negative impact on the school's reputation.