Context switch in computing language is a process of storing the state of a process or of a thread, so that it can be restored and execution of the process is resumed from the same point later.
Multi-tasking requires context switching which does not give productivity gains. There is an increase in the level of frustration for people involved in working on these tasks developers and delivery manager waiting for the tasks to be completed. The greater the level of context switching, the greater the loss in productivity.
Cost of switching context is a form of waste, this waste can be minimised by minimising the number of times a developer switches context.
Book: The Art of Doing Twice the Work in Half the Time by Jeff Sutherland
Book: The Principles of Product Development Flow: Second Generation Lean Product Development by Donald Reinertsen