is a fully managed service that makes easy to develop GraphQL APIs, so that application can access, manipulate and receive real-time updates from multiple data sources ( resolvers will grab and aggregate sources like Elasticsearch, DynamoDb, Lambda)

it automatically scales, provides server-side data caching ( without need of managing cache clusters )

AWS Serverless Hero Yan Cui has some very interesting articles ( and courses ) about AppSync, defenetely worth a look!