They substitute for macro lenses. They are filters that you can screw onto your dslr camera. They extend the focus range, allowing you to move in for closer pictures. The results aren't as good as with a real macro lens, but filters are more easily stored and much cheaper. Macro lenses cost around $500, while I got my macro filters for $15.
You can buy macro filters with different magnifications such as 1x, 2x, 4x, 10x, and others. You can use each filter on its own, or combine them for closer magnification. I have a 1x, 2x, 4x, and 10x that I use combined together.
All of these pictures came directly from my camera and have not been edited or cropped.
Without macro filters.