Platform Specific Models are indispensable for the actual implementation of a system.
Say for example that you want to implement an online shop.
The system will need to store information regarding users, goods, credit cards, etc.
You might decide that you will use for this purpose an Oracle database.
For this to work you will need to express your concepts (e.g. the concept of an user) in a relational model using the Oracle's SQL dialect.
This Oracle's specific relational model is an example of a Platform Specific Model.
The term Platform Specific Model is most frequently used in the context of Model Driven Architectures.
See also Platform Independent Model.