M2 has an XBee-compatible form factor socket on the top of the processor board. In other words: two 2mm pitch x 10 pin headers spaced 22mm apart. You an read more about XBee modules on Wikipedia​
There are LOTs of modules that fit this socket, with many different functions. WiFi, BLE, GSM, LTE, Ethernet, etc.
When connecting a XBee-compatible form factor socket to M2, makes sure that is is oriented so that pin 1 (+3.3V) is at the corner nearest the USB socket as shown here:
For an image showing the pin names of the XBee socket on M2, click here.​
M2 has a USB connector that can act as either a HOST or DEVICE depending on your code.
M2 uses Native USB port only. To print something to the Serial monitor, you'd need to use this command:
SerialUSB.print("Something");