The nucleolus is located approximately in the center of the nucleus. It is a specialized region that plays an important role in protein synthesis.
Protein synthesis is the process of making a new protein. Many organelles are involved in protein synthesis. The process begins in the nucleolus, but also requires the ribosome, endoplasmic reticulum and golgi apparatus. You can also even say that the cytoplasm is required in addition to vessicles which transport the proteins during the process.
The main function of ribosomes is to make proteins. Protein synthesis requires 3 types of RNA. It requires messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA). The job of the nucleolus is to make rRNA which helps in the production of ribosomes (which are needed to make proteins).