I suppose that you refer to the Lightstreamer-server-side of the adapters (i.e. the Proxy Adapter). Its job is to act as a RPC stub.
In fact, the Proxy Adapter on one side and LS Remote SDK library on the other side work like a RPC layer, transferring the requests from LS Server kernel from the Proxy Adapter to the Remote Adapter (although the interface from LS Server kernel to the Proxy Adapter and the interface between LS Remote SDK library and the Remote Adapter are slightly different).
Hence the LS Server kernel sees the Proxy Adapter as a normal Adapter, but in fact its requests are fulfilled by the Remote Adapter.

To do this, the Proxy Adapter performs the connection with the Remote counterpart and transfers the invocations done by LS Server kernel into messages for the Remote Adapter, and transfers back the messages from the Remote Adapter to LS Server kernel.
For this reason, the Proxy Adapter, as well as LS Remote SDK library, is provided by Lightstreamer and only requires some configuration.
A longer explanation can be found in this document.