Basel III is a global, voluntary regulatory banking standard of managing liquidity and capital adequacy. The third standard came into existence due to the financial crises which shook the whole world during early 2008.
The financial crisis of 2008 was mainly due to bad loans and over leveraging, for this the Basel II norms were overtake by the Basel III norms.
Basel III is suppose to strengthen the banks capital requirement by increasing bank liquidity and by decreasing bank leverage.
Here is an interesting video which will help you understand Basel III in about 10 mins:
Video Credits: BNP Paribas Fortis