The process of doing actual work and gaining thereby actual experience.  It provides an opportunity for a newcomer to demonstrate the competence and work habits necessary to be accepted by the old-timers (the analogy is to the dues one pays to be the member of a club).

Also sometimes said of a certain obligatory suffering in connection with some endeavor.

For programmers, this is the business of actually writing application code and seeing it through to implementation.  The suffering part could be said to be that period of time one spends as a maintenance coder supporting other people's code.

The expected result of this is the accrual of certain wisdom and perspective with regard to design and execution of software engineering projects.