I have two Arduino UNO, two Xbee shield, and two Twi Xbee (type XB24).
I am very new to xBee and my question is: Can I communicate each other via Wi-Fi ?
The image is of a Series 1 XBee. "Out of the box" the pairs are usually configured to talk to each other. They will also talk to every other similar XBee in range, but it is not relevant here.
The simplest answer is that whatever data goes into the XBee on one side, will appear at the other XBee and vice versa. TX/RX of each XBee will talk to the plugged in Arduino - I'm not familiar with the shield (excuse me, daughterboard) pictured but it will be a pair of digital I/O pins. Any digital data from the first Arduino can then be "read" by the second Arduino and the other way as well.
Note the shield (daughterboard) will have a slider switch. In "program" mode it echoes commands to your PC via USB (the X-CTU programming mode pictured). In the other position, it talks to the Arduino. A link to the shield (daughterboard) specs would help get a more specific answer.