Primary Sector means agriculture sector. In a country like India the main source of income even today is agriculture. Many of our agricultural products are exported by which foreign currency is flowing.
The well-being of other two sectors depends upon Agriculture. If agriculture sector is well with good yields then Secondary Sector which consists of industries run smoothly. Most of the industries are based on agricultural products.
Tertiary Sector means service sector. Today the contribution of Service sector is growing very fastly and contributing very high amount in GDP. In a developing country like India all the three sectors are equally important. In a country like USA Tertiary is very important and then Secondary.