That invention is the place value system of counting and the concept of of "0" to stand in for a non-existent number. It may seem trivial today but it was very profound when the Islamic Mathematical Scholars first developed it.
Not the importance of Zero to mathematics... but the idea that islamic mathemeticians "invented" the concept of zero or it's function in mathematics. Zero as we know it was an invention of the *Hindus* in India about 200AD - it was introduced to the arab culture from india.