Monsoon basically refers to air circulation due to which rains happen. But in the Indian subcontinent (i.e., India, Pakistan and Bangladesh) they are mostly understood as cyclonic winds that bring rain.
By the time you will be going through these lines, monsoon will be upon us bringing a lot of cheer. Entire India awaits these rains as summer would have made us weary and the land dry. Famers too place a lot of hope. It is essential to recharge the groundwater aquifers and for planting of paddy. It also brings water for reservoirs which collect them for irrigation and produce electricity.
India has two monsoons. The first, South West Monsoon breaks over Kerala mostly on June 1. It is brought by winds from Indian Ocean. It slowly covers the Deccan Plateau and then goes over the North and enters into Pakistan around July. The second, the North East Monsoon rises from Bay of Bengal and brings a lot of rain to the entire east coast of India. These rains are mostly caused by cyclones which come with the ferocity of storms and may cause damage too. Nevertheless both Monsoons are welcome.
But strictly speaking, Monsoons happen because land and sea have different capacities to absorb heat. During summers, the land becomes very hot while oceans stay cooler due to convection of heat (warmer water going down and pushing the cooler up). This keeps the water cool and the air above retains high pressure. This difference in pressure causes sea breezes to blow from the ocean to the land, bringing moist air inland. It rises high over the land, cools and turns into water and this water comes down in the form of rains.
These cyclical changes in weather are a bounty from God and an arrangement from Him to provide us sustenance.