A blessing is way to wish good luck for a person. Sometimes, in religious rituals, it is said the God blesses those who are good, or people can bless God. In Judaism, they bless God over wine. The priests can also bless religious objects in order to make them become holy.
The saying "bless you" is commonly used when someone sneezes. This is done because there is a tradition carried by the superstition that a person's soul will come out of their mouth and nose if they sneeze. People believed that saying "God bless you," or "bless you" for short, would make the soul go back inside. They are wishing the person good luck so that their soul is able to come back. 
People also think that if unusual good luck or a miracle comes to them, it is a blessing. For example, if some people wanted a child badly but are having difficulty conceiving, it could be considered a blessing from God if they end up having a child.
To give your blessing to someone is to give permission to them. For example, a man can ask his girlfriend's parents' for their blessing before he asks her to marry him.
Blessings are often thought of as the opposite of a curse.
- "Does your heart stop when you sneeze?". Library of Congress: Every Mysteries. Retrieved August 12, 2020.