RNA interference (RNAi or miRNA) is a method by which the cell regulates gene expression post transcription. RNAi works by having small semi complemetary RNA molecules bind the mRNA transcript in the cytoplasm, before a protein is made and signals for degredation of the transcript. In this way, the cell is able to maintain a homeostasis of protein amount and is able to work quickly to keep it in check.
Dicer and Drosha are the common proteins associated with signaling the transcript for degredation. You can find details on the mechanism here
Researchers have adapted this natural process to work in vitro. They have engineered small RNA molecules that can go in and mimic the miRNA and bind the transcript, signalling it for degredation. This technique is often used in genetic studies where the researchers want to know our gene expression for a single gene and see the effect.