The "5' cap" is a special structure added to the beginning of a messenger RNA (mRNA) molecule during the process of transcription. This cap consists of a modified guanine nucleotide that helps protect the mRNA from degradation and plays a crucial role in the initiation of translation, where proteins are made. Additionally, the 5' cap assists in the binding of the mRNA to the ribosome, the cellular machinery responsible for protein synthesis. This ensures that the genetic information encoded in the mRNA is accurately translated into proteins, which are essential for various functions in living organisms.