This pressure gauge screws onto the bottom of your portafilter with a 3/8" diameter. It will replace the downspout fixture on the bottom of your portafilter (make sure that this unscrews, and is 3/8" before ordering).
The fixture itself is shown in the main photo; the inset photo is a picture of the portafilter with this gauge threaded onto it after the spout fixture has been unscrewed.
This gauge will measure the actual extraction pressure as "seen by" the portafilter basket when you pull a shot. The gauge shows a range between 0 and 16 bars (0 and 230 psi). Espresso extraction should be at about 9 bars.