It's a term of endearment used for someone you love, often a child or a romantic partner.
Here are some examples of how you might use it in English:
* "Come here, mon petit."
* "You're my mon petit, you know."
* "I love you, mon petit."
It's important to note that "mon petit" is a very informal and affectionate term, so it's best to use it with people you are close to.