A resolution (size) of 320 x 240 pixels. This can refer to the resolution of a physical component such as a display or camera sensor, or it can refer to content, such as the resolution of a photo or video file.
QVGA is a common resolution for phone displays, and also for video captured by phones.
Higher resolution offers more detail. QVGA is larger (higher resolution) than QCIF, but smaller than VGA.
VGA is 640 x 480 pixels. QVGA is therefore one-quarter the area, hence the name Quarter-VGA (QVGA).