Self-shipping, or selfshipping, is to ship oneself with a canon character from pre-established media, or with an OC. Selfshippers may draw themselves with their "F/O" (fictional other) or commission others to do so. They may also write fanfiction with their F/O and a self-insert (S/I), though not all self shippers use S/Is.
Some people do this just for fun, while some folks use it as a coping mechanism, and others still have genuine romantic/sexual feelings for their fictional other (F/O) and will forego IRL relationships for their F/O (though this is not always the case!)
As previously mentioned, 'F/O' stands for 'fictional other', a term similar to 'significant other'. Unlike significant others, however, an F/O can be familial, platonic, romantic, or even an enemy. F/Os are usually characters from popular media, like books or movies, and can be anything from villians to background characters. People who have F/Os are aware that their F/Os are not real, but may talk about imagined interactions with their F/Os as if the relationship is real
♥ their F/Os aren't real.
♥ it's not okay or polite t0 yell at other people for pairing themselves up with the same character (though it's alright to not want to interact with others who self ship with the same f/o, if you aren't rude about it)
♥ don't let the fictional relationship consume their life to an unhealthy degree
♥ and don't encourage dangerous relationships
They should be perfectly fine!
Q: but a fictional character can't love you back
A: Yes, that's quite obvious (/nm). F/Os are fictional, they aren't real, and self shippers are aware of that - they recognize that there is a difference between self-shipping and an actual relationship.
Q: It's not fair on a real life partner, though
A: If the partner is fine with it, and the fictional relationship isn't harming the real one or the people involved, then keep your nose out of other people's relationships.
Self-shipping is generally only bad (with some exceptions) when the person neglects their real life for their fantasy, goes so far as to inflict physical/emotional abuse on others because of it, refuses to acknowledge their f/o as fictional, or allows self shipping to fuel mental illnesses or harmful paraphilias (such as necrophelia or pedophilia)