Teleconverters are optics that sit between the lens and the image sensor. They can generally be found at 1.4x and 2x magnifications. They effectively change the focal length of the lens by their magnification factor. A 100mm lens with a 2x teleconverter, for example, will effectively become a 200mm. They also retain the original lens’s working distance–which is great if you’re happy with your current working distance. You also retain the ability to focus to infinity.
However, teleconverters are not without their drawbacks. For one, you are adding additional optical elements–so loss of image quality should definitely be a concern. Another issue you will need to contend with is light loss. As a rule of thumb, you can assume that you will lose 1 stop of light at 1.4 or 1.5x, 2 stops at 2x, and 3 stops at 3x. What this means is that you may not be able to shoot in natural light if you need a high shutter speed.
Teleconverters can also be quite expensive if you’re shooting with equipment that uses electronic aperture control. If you opt for legacy glass, a quality teleconverter can actually be very affordable. Again, do your research and ask around if you’d like to use a teleconverter for macro.
And last, but not least, macro lenses!