Believing in yourself is knowing who you are, what you stand for, what your capable of and can achieve, having desire and drive to do so, and possibly most important of all, knowing what you don’t know, and where your limits lie, and being honest with yourself. It means having the awareness of self to identify things that will trip you up and remove them, the willpower to resist the temptations you cannot remove, and the stamina to persevere when things are hard. I would also argue that true strength of character also requires humility, a willingness to listen to others and take in constructive criticism, while disregarding the rest.
These things combined make up a strong person, and strong people believe in themselves.
This is a complicated question if you think about it.