For the technique to work, the machines work in pairs, with one broadcasting wireless transmissions and another (positioned on the opposite end) measuring them. Since walls and objects reduce signal strength, the receiver can distinguish between empty and occupied spaces to create an accurate map of the area.
These aren't the first robots we've seen that can peer through concrete, but the Cougar20-H surveillance robot that emerged years ago uses a number of sensors, whereas UCSB's creation depends solely on WiFi.
In the future, the researchers believe the technology can be used for search and rescue (it'll certainly be very useful in rescuing people out of collapsed buildings after earthquakes), to determine the occupancy level of a location and as a helper for archaeological digs. Also, these robots can potentially be outfitted with technology that can classify the object (if it's human, or what it's made of if it's not) they're seeing through walls.
The UCSB team, which has been working on this project for years, has just released a video of the robots in action that you can watch below.