Jawan is an informal term for a rank and file soldier of the Indian army. The more formal term Sepoy refers to the specific rank of infantry or private soldier; jawan may refer to soldiers of higher ranks, up to about the level of Junior Commissioned Officers. Being informal, the exact definition of who is a jawan changes from speaker to speaker.
Jawan comes from the Persian جوان (Javān) meaning 'young', and was traditionally used to refer to a young man. This meaning has become secondary in modern times. Most particularly, when you see the term jawan in English works, it will almost always refer to an Indian or Pakistani soldier. It may sometimes also be used to refer to a donkey in the Bengali language, although you will rarely if ever find this usage in English works.