Along with an object we also have holes. On the left you'll see a regular box. On the right, you'll see a box that is a hole. any object can be switched back and forth between an object and a hole. The way you do so is by selecting an object going to the properties section in the top right. Just a reminder, you may need to click this arrow to drop down the properties.

And then you'll see the option to change from a solid to a hole, or from a hole to a solid. Simply click on which one you'd like your object to be. Notice that when I clicked on hole, my object turned into a hole. Now I can click on this one and turn it into a solid. holes are used for removing parts of an object. For example, if I click and drag this hole to be overlapping With my box I can use the group tool to cut away part of that box.

You can also drag either box or cylinder hole from the basic shapes. But any of these objects can be turned into a hole by using a holes along with the group tool you can make very unique shapes