Software development is a series of trade offs between features implemented, the quality of the features, and how long it takes to produce. But, no matter how much effort is put in, one cannot sustainably maximize all three of these without increasing the cost of the project.
For software development, often cost is directly related to the number of people assigned to and working on a project. This concept is often summed up as “Pick Two: Quality, Speed, Features”.
This relationship between quality, features, speed, and cost is illustrated by imagining a triangle with a fixed volume with the volume being quality. One can increase speed to implement and the features, but quality will suffer if cost is unchanged, keeping the volume of the triangle the same.
Frequently cost is assumed to be constant because in most cases the cost is fixed and there is no budget to add more people nor are there people available for transfer from other projects.