A tithe means 10%. In the Bible, God instructs us to tithe, or give 10% of our gross income, to the church. What's unique about tithing, is that does more for us than it does for God. Also, it's the only way in which God says we can test him.
Bring the whole tithe into the storehouse, that there may be food in my house. Test me in this,” says the Lord Almighty, “and see if I will not throw open the floodgates of heaven and pour out so much blessing that there will not be room enough to store it. (Malachi 3:10 NIV)
At the end of the day, God wants us to be in control of our money - not the other way around. God knows how easily money can control us, because at it's roots, tithing is about our heart, not our money.
For where your treasure is, there your heart will be also. (Matthew 6:11 NIV)
Simply put, God wants our first and our best. When you give your first and best to God, the rest is blessed.
Honor the Lord with your wealth, with the firstfruits of all your crops; then your barns will be filled to overflowing, and your vats will brim over with new wine. (Proverbs 3:9, 10 NIV)