siRNA, or small interfering RNA, is a class of double-stranded RNA molecules that play a vital role in the RNA interference (RNAi) pathway, a cellular process that can effectively silence or reduce the expression of specific genes. siRNAs are typically 20-25 nucleotides in length and function by guiding the RNA-induced silencing complex (RISC) to complementary mRNA sequences, leading to their cleavage and subsequent degradation.