CTD stands for Conductivity, Temperature and Depth. The CTD can be lowered from a research vessel on a cable all the way to the sea floor (more than 6 km) and depth of the CTD is worked out by measuring the pressure.
Ocean water properties such as temperature are measured directly, while conductivity is used to tell us the salinity. It also captures water samples at different depths for a variety of chemical and biological measurements made either on the vessel or back in the laboratory.
From this we can work out how the ocean mixes and stores heat, carbon, oxygen, as well as measuring nutrients and gaining information about the habitat of aquatic life.