Pairing development can also be referred to as pairing programming, which is the practice of collaborating on a piece of code in pairs. A pair of developers will write and revise code alternately.
As the name implies, pair programming essentially involves two people writing code together on a single machine. In general, when working in this manner it is a very collaborative process and a lot of communication takes place.
Developers working on a task together are not only writing code together, but they are also planning and discussing the work they are doing. By clarifying ideas and discussing approaches, they are able to come up with better solutions.