Hello Lukasz,
The serial manager is used for communication to the upper layers in the stack. For most thread examples, the serial manager is user by the shell interface and communicates through UART to the OpenSDA MCU.
Since the KW41 only has one UART, you would need to disable the serial manager to have access to the UART. Once disabled, you can use the SDK drivers for the UART module. There are examples for this driver located in <SDK installation>\boards\frdmkw41z\driver_examples\lpuart\.
FSCI is a serial protocol used to control the Connectivity stack through serial commands.
Regards,
Gerardo