GraphQL is a high level communication protocol that can be used as an alternative to REST APIs. Ever since the keynote DrupalCon 2015 in Barcelona the interest in GraphQL has increased in the Drupal community. GraphQL is a great match for Drupal as the use of Drupal as a headless CMS continues to rise.
RESTful interfaces and headless implementations have been on the minds of developers and less technical people alike. This has been the state for a number of years now and frankly the REST hype is getting a bit long in the tooth.
While REST is a technology with merits, the problem is that REST is only used as a baseline technology definition in most cases. There is no specific definition on what a REST API is, that's why many resort to calling them RESTful - to describe that they sort of fulfil the definition. But without knowing exactly what is meant by REST, it's hard to say.