Shipping comes from the word “relationship” and means that fans believe that certain characters are involved in a relationship.
Shipping is a very difficult term to explain seeing as it’s very emotionally charged. Not only from the characters that are supposed to be in a relationship (according to the fans), but from how the fans ardently feel that their ships are real.
According to Wikipedia:
Shipping can involve virtually any kind of relationship — from the well-known and established, to the ambiguous or those undergoing development, and even to the highly improbable and the blatantly impossible. People involved in shipping (called shippers) variously assert that the relationship does exist, will exist, or simply that they would like it to exist.
Shipping can also mean a regular ship, meaning that the fans ride the same ship, as in the pairing they are shipping.
Shipping occurs in every fandom. The sexual orientation of the shipping varies, it goes for monogamous heterosexual relationships, to homosexual ones, polygamous relationships with all kinds of sexualities involved to a very little strange ship where males can get pregnant — don’t ask.
If there’s a pairing to be obsessed about, then you ship it, and depending on what type of sexual relationship that is, then other terms start to pop up. Such as het (heterosexual), and finally slash.