Tithing refers to the act of giving back a tenth of what God has first given to us because the tithe belongs to Him. God calls us to give the tenth (10%) of our income back to 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
The act of offering is a reflection of a heart of gratitude towards what God has blessed us with. We can also give because of God’s goodness and His faithfulness in our lives.
7 Each of you should give what you have decided in your heart to give, not reluctantly or under compulsion, for God loves a cheerful giver. 2 Corinthians 9:7 NIV