What is the 700 times reward?

The Reward for Sadaqah

In Islam, the blessings associated with regular donations, known as Sadaqah, given solely for the pleasure of Allah, are rewarded with a multiplication of up to 700 times. The analogy used to describe those who contribute their wealth for Allah's cause is that of a single grain of seed; it grows into seven ears, with each ear containing a hundred grains. Furthermore, Allah bestows His rewards on whom He chooses.