To be certified ISO 9001 is that to assure the quality of product or the quality of service. Can you explain how?

Actually neither. ISO certification is about standardization of the process. The good quality of either the product or the service or both is a natural (by)product of good practices that are ducumented and reproducible. The best thing is that continual improvement is built in so that the end product and/or service  automatically and continually gets better

1. Increased Efficiency

2. Increased Revenue

3. Employee Morale

4. International Recognition

5. Factual Approach to Decision Making

6. Supplier Relationships

7. Documentation

8. Consistency

ISO focuses on both product and services. If you are a service provider it will help you enhance the quality of the service and if you are a product manufacturer or developer it will effect the quality of the product.

W.r.t. how it will, documentation is an important aspect of ISO. Write what you do and do what you have written. Consider you have to go to a hicking trip. At the packing time you pack whatever you remember to be important for the trip at the time of packing. This may make you forget something as at the execution time you are depending on your memory. But if you take some time before the actual packing you write down the things you require after a few minutes of brainstorming and when executing the packing activity you use the checklist there is no way you will miss anything. So the quality of the execution of the packing excercise has improved.