When you want to connect an array of data to a set of DOM elements, you use D3's data binding methods. The data() method joins your data array to selected elements. However, if there are more data points than DOM elements, D3 needs to create new elements for the extra data. This is where the enter selection comes in. The .enter() method returns a placeholder selection representing the data points that do not have corresponding DOM elements yet. You can then use .append() to create new elements for each of those data points. In the code above, the array numbers is bound to any div elements in the body. For each number that does not have a matching div, .enter() creates a placeholder, and .append("div") adds a new div for each number. The .text() method then sets the content of each new div to display its corresponding value.