What is RNAi?

At its core, RNAi is a method of gene silencing. It works by activating the RISC complex, which is used to chop up parts of mRNA into fragments so that the gene is not expressed. 

To help understand this, think of an assembly line where each person is following the instructions on how to make something. Now, suppose that someone came along and cut up the instruction manual; now, no one knows how to make the product. The same thing happens with RNAi, the instruction manual, or in this case, the mRNA is chopped up, so that the workers (ribosome) can not produce the enzyme the gene is responsible for.