The nucleus is usually a round structure found in the center of cells. The nuclear envelope is wrapped around the nucleus, and it controls what goes in and what goes out of the nucleus.
The nucleus is called the control center of the cell because its job is to turn on and turn off the processes of the cell, like when to make a copy of itself, or when it needs to make more proteins.
The nucleus also contains the cell's DNA, which are the instructions for the cell.
Where does the nucleus tell the ribosomes to go? Usually, it starts with the endoplasmic reticulum.