A Client refers to a company or someone that you carry out work for.
Most modules in Assignar link back to the Clients module. It is a key building block in linking of data. It assists you being able to easily distinguish what work is or has been carried out for a period of time.
You'll need to have clients in the system as that is what everything relates back to. For example, you won't be able to assign your workers or have timesheets submitted without Clients set up.