Mobile Backend as a Service, or short MBaaS, are cloud products that bundle up existing databases, interfaces and feeds into accessible APIs that can be exposed to developers or to make mobile apps with. Most MBaaS platforms are heavily focusing on the enterprise market by bringing on-premise storage solutions out into the Internet. Thus, naturally, MBaaS solutions are concerned with adding a high degree of security to this previously not exposed data so that the APIs created with the MBaaS are as secure as any other SaaS application that company uses in production.
MBaaS solutions are part of the API-first paradigm where data from any origin or system should be securely accessible via the Internet in a standardized format, called the RESTful API. Thus it is middleware and is concerned solely with making data as easily accessible to application front-ends, leaving the actual development of mobile or desktop applications to front-end frameworks.