As the name suggests, this is where the development of the software takes place. It is the first environment where developers write the first lines of code and do all the code updates.
This environment usually consists of a server that is shared by several developers working together on the same project.
Therefore, when multiple developers are involved, a developer would have a copy of the source code on their own machine and then changes are submitted to the repository which are then integrated into a branch.
A number of preliminary testing takes place at this stage before moving on to the next stages.
For example, if developers have adopted a continuous integration methodology for their software development process then several changes daily would be committed to the branch.