I know what it means, but what does it really MEAN to swear an oath on or by something?

If you swear on the Bible or on your mother’s gray hairs or your father’s grave, or you swear “by God” or on the heads of your children, what are you really saying?

Are you saying that your oath is as strong as they are, or as your devotion to them is?

Are you saying that if you fail to keep your oath, those precious things are in some kind of jeopardy?

Are you saying that the power of those things in your life will somehow lend strength to your promise or your ability to fulfill it?

What exactly is the difference between saying “I swear” and “I swear to God”? What exactly is the difference between taking an oath with your hand on the Bible and without it? Sure, it makes your words sound more serious, but what does it actually mean?