Relation between People and Time to develope

The relationship between number of people and time to develop an application is not linear, what do you understand by this statement?

The relationship between the number of people and time to develop an application is not linear. It is not as simple as a 120 man-day project can be developed by 1 person working for 120 days or 120 people working for 1 day. The communication and coordination overhead plays a very significant role.