This pressure gauge screws onto the bottom of a portafilter. It will fit the portafiltrer directly if the screw diameter is 3/8". When in place, it will replace the downspout fixture on the bottom of your portafilter (make sure that this unscrews, and is 3/8" before ordering).
If your portafilter does not have a 3/8" connection, this won't fit it directly; you'll need to make a trip to the hardware store to get the coupling pieces that allow you to make it connect. You might also want to use some of our teflon thread-seal tape to make those couplings air-tight.
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.