It is what the two words 'out of' sound like when spoken fast. For example, if you are angry with someone and you want them to leave your house immediately, you might shout 'Get out of my house!' . This will sound like 'Get outta my house'. You may see this form in song lyrics or film subtitles, but it is not proper English, so you should avoid writing it. You need to write the full form of the two words - 'out of.'

outta means 'out of'