SAP released a new development concept a while ago which enables building new applications on the SAP Cloud platform using the Cloud Foundry runtime environment.
SAP has currently 2 cloud platforms available - the standard 'classic' NEO platform which is built upon a proprietary technical layer and the newly 'Cloud Foundry' platform which uses the 'Cloud Foundry' open standard as underlying technical layer.
CF supports a number of default pluggable 'run times' which basically deliver a technical run time layer using certain technical standards and programming language(s). There are eg run times for 'JAVA' and 'node.JS'.
SAP still provides access to an in-memory HANA database and the necessary API & services to handle the various integrations with SAP & non-SAP systems.
The difference though is mainly situated in the way applications are developed and which programming languages can be used for that.
The NEO platform provides the 'classical' XS programming model to build applications running on HANA.
The SAP CF platform supports multilple run times and hence provides a vary different application environment and paradigm. The underlying architecture also forces the usage of separate isolated containers which practically means that every part of a solution runs independently - eg your web application and API microservices. This also implies that one can use different programming languages to develop the different components.
Bringing this all together in the SAP webIDE required a new way to defining & building applications ... hence the 'Multi-Target Application' concept.
An MTA project basically allows for the definition of different sub components and ensures those can talk with each other and are eventually build & compiled into 1 build archive which can be deployed into the cloud.
This concept is still quite new and still requires quite some manual labour and usage of command line tools to get everything deployed and working properly in the cloud.
But it is nevertheless the only way forward for any new SAP Cloud development.