A "chronometer" is a catch-all term for any object designed to tell time. Pocketwatches, grandfather clocks, water clocks, and more can all be called "chronometers". However, when studying navigation, particular focus should be given to marine chronometers.
Telling time onboard a ship is difficult for many reasons. Determining the local time can be done via sunrise and sunset tables, but determining the time in a more absolute way (for instance, the time in London) is very difficult. Sand glasses are too small, too inaccurate, and must be turned too often. Pendulum-based clocks do not keep accurate time as a ship pitches and rolls on the waves. Water clocks were likewise inaccurate, and often quite heavy. Harrison's style of windable clock was the eventual answer to these problems.
Harrison Clock, Sand Glass
Harrison's clock in 1741 was the first accurate marine chronometer, but attempts were made as early as 1673. Mechanical chronometers mostly went out of use in the late 20th century as electronic clocks became cheap and accurate.