During an expansion of an industry you can assign more people to work there. Under ideal conditions, those people would immediately begin to work in the new industry, turning out the desired products. In reality, however, the factories usually do not yet exist, when you make the labor assignment. This creates what is known as structural unemployment within the economy. Jobs are available, but they are the wrong jobs for the available workers. To rectify this problem examine Basic Industries to begin rapid construction of the required factories, thus relieving the unemployment.
Until the jobs are available colonists are sitting around collecting welfare payments and eating up food. This is highly undesirable.
One of your goals as the owner of the planet is to keep the everybody working, keeping a careful watch over each colony to maintain maximum production and unemployment low.