Endoribonuclease that plays a central role in postnatal germ cells by repressing transposable elements and preventing their mobilization, which is essential for the germline integrity. Acts via the piRNA metabolic process, which mediates the repression of transposable elements during meiosis by forming complexes composed of piRNAs and Piwi proteins and govern the methylation and subsequent repression of transposons. Directly binds methylated piRNAs, a class of 24 to 30 nucleotide RNAs that are generated by a Dicer-independent mechanism and are primarily derived from transposons and other repeated sequence elements. Strongly prefers a uridine in the first position of their guide (g1U preference, also named 1U-bias). Besides their function in transposable elements repression, piRNAs are probably involved in other processes during meiosis such as translation regulation. Not involved in the piRNA amplification loop, also named ping-pong amplification cycle. Acts as an endoribonuclease that cleaves transposon messenger RNAs (By similarity). Component of the meiotic nuage, also named P granule, a germ-cell-specific organelle required to repress transposon activity during meiosis. The PAZ domain specifically recognizes binds the 2'-O-methylated 3'-end of piRNAs. The MID region is required for recognition of uridine in the first position of piRNAs (g1U preference, also named 1U-bias). Methylated on arginine residues; required for the interaction with Tudor domain-containing protein and subsequent localization to the meiotic nuage, also named P granule. Belongs to the argonaute family. Piwi subfamily.