What does "swear to you" mean?
"I swear to you" (or similarly, "I swear to God") is an informal expression used when someone wants to strongly or sincerely express their intent to do something. It can also sometimes be used to make a promise. In this case, Peter Quill is using this expression to show that he is serious about his threat to hurt Spider Man if they don't help him. Ex: I swear to you, everything I just told you is true. Not a single lie. Ex: Stop gaming or I swear to you, I'll cut off the internet.