Manat. A vow is like a promise. Suppose you ask God for something and promise in your heart that if God gives me what I ask for, I will fast. Now the fast you want to keep is a vow. If God gives you what you want , then you must keep your vows. Otherwise you will be a sinner. And if you don't get what you want from Allah, you have to be patient and ask for that thing more and more. GOD will surely fulfill it one day. Hope you understand.