peat is like grinded up wood/leaves/soil that's aged. for aquarium use, you can buy them in a dry brick block form, it will expand and break loose in small particles in water.
to use peat in your filter, put it in a filer media bag. it will tint your color, very natural for amazon biotope. also peat contains something (forgot the term right now), but basically, it not only lowers ph, creates natural water color, but there are microscopic stuff that affects your fish mentally and physically.
(that sounded kinda funky, sorry im brain dead. but bottom line is peat is good stuff!)