A Stratis Full Node is a computer running the Stratis Full Node software. The Full Nodes act as the foundation for the Stratis Platform. The Stratis Full Nodes run the C# Stratis blockchain and together form the C# Stratis network. Blockchain technology is built up of many components. The Full Nodes make the most of this by being modular: they allow someone using the Stratis Platform to pick and choose which components they want for their application. Think of it like blockchain lego: the Full Nodes provide the basic foundation and allow you to stack up the different components of the Stratis Platform in whatever way suits you.
The flexibility of the Stratis Full Nodes makes them very powerful. All platforms require a user to accept some compromises when building an application. There will always be limitations to what is possible. The Stratis Platform strives to eliminate as much compromise as possible by offering as much flexibility as possible. The Full Nodes allow a user to utilise all the features of the Stratis Platform and moreover allows them utilise them structured however they want. If someone wants to build an application which relies on smart contracts, they can do so from the Stratis mainchain itself. However, they are limited by the speed and nature of the mainchain and thus possibly being forced to accept some compromise in building their application. Fortunately, they could deploy to an existing sidechain better suited to supporting the contract. Or, given the modular nature of the Full Node foundation for the Stratis Platform, they could create a new sidechain with characteristics to suit their application and then deploy their smart contracts on that chain instead.