Remapping feature allows you to select which GPIO line of your device will function as the RTS line of a serial port. This is done through the ser.rtsmap property. In the same manner, the CTS line of a serial port can be mapped to the interrupt line of your choice (see ser.ctsmap).
Not all devices support this feature. To find out if your device allows RTS/CTS remapping, refer to its platform documentation (for example, EM1000's is here). This topic will also present the default mapping for CTS and RTS lines of your device.
Note that on some devices you need to explicitly configure CTS lines as inputs and RTS lines as outputs.