Basically it mean a SONG that fits your characters personality..

well you chose a song that fits ur characters personality and history for example :

if u had a character that u want to be a hero like my future character ill use the song "Bet on it" to show he wants to be stronger and wants to help etc.

i hope that helps

~ReikaOfOhu~

P.S
do not use the example im using it

You chose the song you think sums up your character the best. It can be anything at all. It can be an orchestral film score with no words (a noble warrior character with the Fire Emblem Theme from Super Smash Brothers Brawl,) or it could be in another language (a mysterious wandering traveller with Reset from Okami.)

Basically, ask yourself "if you had to describe your character, but you can only use a song, which would it be and why?" Then you got your theme song.

so like if it's a brave char you can use a song about someone brave?

Basically a song that describes the char in history, personality, etc.