What is the Constitution referring to when it states that "actual Enumeration shall be made within three years after the first Meeting of the Congress (1787) of the United States, and within every subsequent Term of ten years"?

It refers to the fact that the House of Representatives must be composed of members who are selected every two years by the people of the states.

This constitution also also facilitates the processes in which congressional districts are to be shared among the states.

This constitution states that every 10 years, every grown individual in the US must participate in a survey in relation to individuals relocating as well as the homeless. These surveys are then used by congress to determine how many house of representatives are to come from each state and how federal resources are to be shared among the states.

This constitution also refers to the outlined operating rules for the members of the House of Representatives.

The constitution was referring that:

To ensure that states were represented in proportion to their population, there is an establishment that every 10 years, every adult in the country must answer a survey. It is a monumental task because there is movement of people as often as they do and when some people have no homes at all.

On the basis of these surveys, Congress must determine how many representatives are to come from each state. At least one representative is required from each state. Also, it determines how federal resources are to be distributed among the states.

It also provided that each state shall have at least one U. S. House member.

The Constitution set the number of House members from each of the original 13 states that was used until the first census was completed.

This enumeration is what is now known as the U. S Census.