The various types of ‘rocks’ like Marine Pure, Seachem Matrix, Brightwell NO3 bricks, basically all do the same thing and are designed to aid the biological filtration (primarily nitrate removal) from your tank. Most relay on anaerobic denitrification as the prime method, but it depends on flow etc.
They have tiny holes for the bacteria to populate
They do the same job as live rock but can be used in the sump so their not in the display.
If your nitrates are ‘really high’ you may be best starting with water changes to bring them down as well as increasing your biological filtration methods.
You could also look into carbon dosing (Nopox for example) which is a another method and can be used at the same time.