If you're annoyed or angry with whoever you're talking to it basically means whatever, just get the hell out of your face, shuttup, and leave you the fuck alone.
If you're not mad it means: Ok, sure I'll go along with that; or I'll do it later.
Can also be used with affection to a loved family member in place of "I love you too," in some cases.
Teen: (goes downstairs to get a snack and drink)
Grandmother: (limps in with cane) It's your day to clean the kitchen
Teen: (in an indifferent tone) If you say so (goes upstairs with snack and drink and continues to play on computer)
Teen: (thinking to self) I can't stand that bitch, she gets on my fuckin' nerves! She needs to turn on the damn air conditioning cuz it feels like a sauna up in dis bitch!
Grandmother: (downstairs in kitchen)(limping slowly with cane to get a drink)
(daughter and father finishing a phone conversation)
Daughter: What are we doing when you get off work?
Father: I don't know sweetheart, probably nothing (obviously kidding). I have to go now, I have a customer that I need to get back to. I love you.
Daughter: Daad come on!
Father: I have to go now, I'll call you back later. Bye, I love you, and I'll see you when I get home.
Daughter: If you say so, bye! (hangs up phone and continues to watch TV)