The Thar Desert provide the majority of India's rainfall.. Does this mean that the thar desert prevents rain

That statement doesn't make sense. By definition, deserts are arid and get little or virtually no rainfall.

http://en.wikipedia.org/wiki/Thar_Desert

No, the Thar Desert does not prevent rain in India. In fact, the Thar Desert is an arid region with very little rainfall. However, it does play a role in the monsoon season in India, which brings the majority of the country's rainfall.

During the summer monsoon season, moisture-laden winds from the Indian Ocean blow towards India. These winds are driven by temperature differences between the landmass and the ocean. As they approach the Indian subcontinent, they hit the Western Ghats and the Himalayas, resulting in orographic uplift. This uplift forces the moisture-laden air to rise, cool, and condense, leading to the formation of clouds and eventually rainfall.

The Thar Desert acts as a heat sink, which means it absorbs heat during the day and radiates it back into the atmosphere at night. This creates a low-pressure zone over the desert, which attracts the monsoon winds. As these winds move over the Thar Desert, they pick up heat and moisture, which contribute to the monsoon rainfall in other parts of India.

So, while the Thar Desert doesn't directly provide rainfall, its presence and characteristics influence the movement of the monsoon winds, which in turn help bring rainfall to other regions of India.