The life cycle of the xenomorph is not unlike an insect involving many stages of development beginning life as an egg laid by the queen. It hatches as a “face hugger” when suitable prey is nearby.
The huggers job is to attach itself to the host and implant an embryo in the host which will, in a short time, become a xenomorph infant known affectionately as a “chest burster”. This is known as the gestation phase. The infant is then believed to cocoon or molt and become an adult though little is actually referenced in the films.
Adult xenomorphs are lethal killing machines designed to protect and serve the queen of the species much like a beehive or ant colony.