Originally, purity refers to anything that is pure, clean and free from harmful substances.
In science, we say that something is pure if it doesn’t contain any other element.
Nowadays this value is mostly religious. In christianity, only God is perfectly pure. If a believer wants to be worthy of His love, he has to purify himself.
In most religions there’re purification rituals that are supposed to help believers to get closer to their god. Those rituals can be done through deprivations, or with water, fire or sacrifices.
In order to be pure, you have to wash out your sins, stop your bad habits, remain chaste...
Purity is often associated with chastity (see chastity), especially for women and young girls.
Purity culture is a religious tendency but even some non religious people believe in it, and it impacts mostly women : a woman has to be pure, modest, she has to dress properly so she won’t attract men, she has to be a virgin until she gets married... (see conservative). This purity culture was born because of patriarchy, it doesn’t really impact men who are freer to do whatever they want with their bodies than women. Women must remain pure, but no one will question a man's purity.