The subject is an operation of mail handling called "casing." The camera was placed so that two men can be seen demonstrating the method. The demonstrators, with stacks of letters in their hands, place them in one of many apertures built into a cabinet or "case."
Two employees of the Post Office Department are removing stacks of letters from the aperatures in the "case" or cabinet and tying them into bundles. They then throw the tied envelopes into the appropriate sacks.
The camera shows the action of two postal employees as they remove mail sacks from the semicircular rack where they are placed so they can be filled with mail for their respective locations. The two postal employees stack the mail sacks on a cart in preparation for dispatch elsewhere.