When a camera detects motion (for instance someone moving) it can respond in several ways

Change Camera

In Options ~ Main Display you can specify Alert and display on motion so that the main display window (large window on upper left) will change to display the image from a camera showing motion.

You can also specify that the main display will cycle through any cameras that detect motion with Automatically loop through cameras and the option Show cameras detecting motion.

Record from cameras that detect motion

In Options ~ Cameras ~ Properties you can select if a cameras will use motions detection and the sensitivity of this detection. When this option is selected video will only be recorded when motion is detected.