The blind filter is used for backflushing on espresso machines that have a pressure release mechanism. This is a standard - and necessary - part of cleaning your machine to keep the channels in the group clear, which makes the extraction pressure uniform.
This is a 58 mm. blind filter that will fit into your portafilter handle of your espresso machine in place of the brew basket. As described in our article on keeping the group clean, (click here on this parenthetical comment to read it), the blind filter is put into the group handle of the espresso machine with some Cafiza powder to backflush the group channels. We recommended 5 cycles of 5 seconds each for a proper backwash, done at the end of each day.
Note that this is for a 58 mm. portafilter. This is the standard size for commercial espresso machines, but please make sure that this is right for your espresso machine prior to ordering.