Shorty also shortie (shôr'te)
noun (plural 'shorties')
1. A person short in stature.
2. A thing of less than average size, length, extension, or duration.
-- American Heritage Dictionary
Shorty is urban slang for a girlfriend, or any girl, often used in rap and hip-hop music.
Originally 'shorty' referred to a young person or newcomer to an activity -- for example, a younger boy just starting to play basketball with the older kids. It could be used when speaking to younger siblings or any younger acquaintance. It might be a term of affection or entry-level acceptance, or if used when talking to an older person, it could be a form of teasing or mocking.
When referring to a female the sense in which shorty is received will depend mainly on context -- the same as 'baby' can be obnoxious (when shouted at a stranger) or a term of endearment. Used casually in reference to someone who is not your steady girlfriend, it will make you sound like a player. Be warned, if you are learning about slang from reading nodes on E2, you will not sound like a cool rap-star millionaire player dude, but a geeky loser nerd.