Job Work itself explains the meaning. It is processing of Goods supplied by the principal for further processing.
Section-2 (68) defines Job Work as “any treatment or process undertaken by a person on goods belonging to another registered person.”
The one who does the said Job Work would be termed as “Job Worker”. The Job Worker is responsible for processing the goods provided by the principal, but it's important to note that ownership of the goods remains with the principal throughout the process.
There may be some reason for goods is being sent for Job Work like Capacity Constraints, Limited Resources, Specialized Equipment, Cost-Efficiency, Time Savings & Focus on Core Competencies.