A T-Mount (or T2 mount) is an adaptor that enables an accessory such as a microscope/telescope adaptor, slide copier, or mirror lens to be fitted to a wide range of 35mm and Digital SLR camera bodies.
They also have a variety of other applications

T-Mount Inserts Fittings for T-Mounts to turn them into macro mounts

T2 Mounts Fit a T-Mount accessory such as a microscope adaptor or digiscoping adaptor to your DSLR

T Mount filter thread adaptors Fit an T-Mount fitting accessory onto the filter thread of your camera lens

T Mount Extension Tubes Use with T-Mount microscope adaptors to increase magnification. Use as a macro set up with an enlarging lens.