In molecular biology, messenger RNA (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein. source 
The vaccines, one of which was authorized for emergency use by the U.S. Food and Drug Administration last week, rely on the genetic instructions known as messenger RNA (mRNA). It prompts cells to make a SARS-CoV-2 protein that trains the immune system to recognize the virus. source
Research is under way on the effectiveness of using mRNA to treat many other diseases, including some cancers. Science Magazine’s article provides some information about that research.