This is the Thai mortar and pestle, it's much larger than the western version. In the west the mortar and pestle are used to crush only fine spices, but in Thailand cuisine, many ingredients are crushed in this vessel, chillis, papaya and sauces are all mixed and created directly in the mortar. 'bok bok' is the name used to describe the action of crushing ingredients in a mortar, mimicking the sound.
The mortar is made of clay, 15cms (6 inches) high, while the pestle is usually made of hard wood, 30cms 12 inches long. You can obtain both from a Chinese or Asian supermarket.
To crush ingredients, raise the pestle and bring it down in repeated blows against the food using the weight of the pestle to help you. This is a very different action to the grinding action used with a western pestle.