However, as with any dating technique there are limits to the kinds of things that can be satisfactorily dated, levels of precision and accuracy, age range constraints, and different levels of susceptibility to contamination.
Radiocarbon dating is especially good for determining the age of sites occupied within the last 26,000 years or so (but has the potential for sites over 50,000), can be used on carbon-based materials (organic or inorganic), and can be accurate to within ±30-50 years.
Materials that originally came from living things, such as wood and natural fibres, can be dated by measuring the amount of carbon-14 they contain.
For example, in 1991, two hikers discovered a mummified man, preserved for centuries in the ice on an alpine mountain.
It’s not absolutely constant due to several variables that affect the levels of cosmic rays reaching the atmosphere, such as the fluctuating strength of the Earth’s magnetic field, solar cycles that influence the amount of cosmic rays entering the solar system, climatic changes and human activities.
Among the significant events that caused a temporary but significant spike in the atmospheric carbon-14 to carbon-12 ratio were above-ground nuclear test detonations in the two decades following World War II.
A detailed description of radiocarbon dating is available at the Wikipedia radiocarbon dating web page.
Bottom line: Radiocarbon dating is a technique used by scientists to learn the ages of biological specimens from the distant past.
There is a small amount of radioactive carbon-14 in all living organisms because it enters the food chain.
Once an organism dies, it stops taking in carbon-14.
The carbon-14 it contained at the time of death decays over a long period of time, and the radioactivity of the material decreases.
Radiocarbon dating was the first chronometric technique widely available to archaeologists and was especially useful because it allowed researchers to directly date the panoply of organic remains often found in archaeological sites including artifacts made from bone, shell, wood, and other carbon based materials.
In contrast to relative dating techniques whereby artifacts were simply designated as "older" or "younger" than other cultural remains based on the presence of fossils or stratigraphic position, 14C dating provided an easy and increasingly accessible way for archaeologists to construct chronologies of human behavior and examine temporal changes through time at a finer scale than what had previously been possible.