A T-ring is used to connect your DSLR camera to a telescope.
You select the T-Ring which matches your camera type in much the same way you need to get the correct lens to attach to it. (Nikon, Canon EOS, Sony etc)
This BINTEL T-Ring is for Canon DSLR EOS camera bodies. If you have a Canon mirrorless camera, check out this T-Ring.
One end of the T-Ring connects to your camera body in place of the lens and the other end connects to the telescope. Some telescopes may already have a "T-thread" connector while others may require a camera connector. Examples of these include: