It is a 191-amino acid, single chain polypeptide hormone that is synthesized, secreted by the somatotroph cells within the lateral wings of the anterior pituitary gland. That's the definition by Wikipedia.

Here is a definition in a layman's terms.
HGH is a hormone that the pituitary glands in our brain creates. It is also known as the youth hormone, because it  is responsible for the height increases, calcium retention, increases in muscle mass and the stimulation of the immune system among many others. The problem is, after a certain age HGH secretion starts slowing down.