A platform-specific model is a model of a software or business system that is linked to a specific technological platform (e.g. a specific programming language, operating system, document file format or database).
Platform-specific models are indispensable for the actual implementation of a system.
For example, if a business needs to implement an online shop, then their software system will need to store different kinds of information: available goods, user info such as credit cards, etc.
The designer might decide to use for this purpose an Oracle database.
In Model-driven architecture, a platform-specific model is where the design of the model is constructed with the intended execution-platform driving design choices.