To pledge is to give one's solemn promise, oath, vow to abstain from something, to give something, or to do something. For instance, when we say the Pledge of Allegiance in front of the US flag, we make a vow to do whatever is necessary to be loyal and patriotic to our country. Or, you can pledge a monetary sum to a charity or foundation. Or, you can pledge to stop drinking alcohol or using drugs. As an aside, Ecclesiastes 5:5 - "Better not to vow (pledge) than to vow (pledge) and not pay."