Rainfall

monsoon (noun): the period of rainfall in the Indian  subcontinent, approximately from June to early October

                          

That dictionary entry does not really satisfy one's curiosity, does it? It tells you only what the word "means". It does not tell you "what" the word means. It is a reference to the events that cause the advent of the rainy season in the Indian subcontinent, which affects all of  us. Normal rainfall in the season has a positive bearing on the economy of the entire region. 

Most of us know this. For those who don't, a layperson's explanation follows...

The Indian subcontinent is the only area that has a time period in which there is rainfall, while many regions of the world have to keep track of the period in which there is no rainfall. Greater north-south and east-west geographical extent plus altitude variations are the special features of the subcontinent leading to varying levels of rainfall.

Causes
There are many causes for the advent of rainfall, which may appear to be disconnected but are direct reasons for the same. The summer season heats up the subcontinent and hence, the ever-moving column of air above it. The air becomes less dense, that is, it starts occupying a larger volume than earlier, though the total amount of air remains the same. This creates a low-pressure area in the subcontinent.

The Indian Ocean, the Arabian Sea and the Bay of Bengal do not get as hot. (Land gets hot faster than water.) So the air above them, too, does not heat up much. The air pressure here is more as compared to that over the land. So, air follows its general tendency - that of moving from an area of higher pressure to an area of lower pressure. As it moves towards the landmass, the air above the water bodies collects moisture. It precipitates or condenses in the form of rainfall on the subcontinent. However, for precipitation to take place, the moisture laden clouds have to come into contact with cooler air on land, which in turn depends on the forest cover and altitude.

That is the perfect set-up. 

Disturbance   

Human activity can disturb this setup by causing deforestation and pollution.

The air column over the subcontinent is being replaced by carbon-dioxide and methane. Methane absorbs twenty times as much heat from sunlight as does carbon-dioxide. The air over the water bodies tries to equalize the air pressure as soon as the difference occurs and moves over the subcontinent too soon, causing irregular or scanty rains.

If this part of the world suffers from seasonal changes, so will the other regions. World climate is interrelated and inter-dependent. Also, pollution caused in one country can affect another and make all regions suffer. So, we have another good reason why pollution should be reduced.