Our organs are constituted by cells: the heart by cardiomyocytes, the liver by hepatocytes, the skin by fibroblasts and so on.. all these cells have a different behaviour and characteristics depending on which DNA parts are expressed and, consequently, which proteins are more present inside the cells. In our case, the cardiomyocytes (cells of the heart) express the proteins that led them beat, while the hepatocytes of the liver express proteins that help them to metabolize. In our body cells already know which genes must be expressed.
In the lab we can direct a cell to express one protein instead another using transcription factors, molecules that can bind the DNA and control which genetic part will be expressed. Metaphorically, what they do is reach the DNA of the cells and tell him:
“Hey man, this cell needs to beat otherwise Marilù cannot perform the experiments! Let’s synthesize some channels and myosin chains to allow the cell contracting!”