A carbon flux is the amount of carbon exchanged between Earth's carbon pools - the oceans, atmosphere, land, and living things - and is typically measured in units of gigatonnes of carbon per year (GtC/yr). A gigatonne is a tremendous amount of mass, roughly twice the mass of all humans on Earth combined, or the mass of about 200 million elephants!
These carbon pools contain enormous quantities of carbon and exchange this matter in various ways. The consequences of this exchange will not be explored deeply on this page, and can instead be read in more detail here.
The Earth's carbon is exchanged globally in what is known as the carbon cycle. This cycle exchanges immense quantities of carbon each year, with values shown in Figure 1 below. (Remember, each value in this figure represents 1 gigatonne, the mass of 200 million elephants.) The carbon cycle balances almost perfectly naturally, however when humans introduce carbon that was originally buried underground, this introduces an imbalance, as shown in red text in Figure 1.
There are two main natural exchanges, which make up the natural carbon cycle. This page only discusses the quantities of carbon exchanged in each, however the main article link above explains exactly how these exchanges take place.
These exchanges both occur on very different timescales, with the land cycle occurring at a high rate while the ocean cycle is much slower.
The human exchange of carbon is mostly a one-way street, as fossil fuels are extracted from deep underground (where they have essentially no effect on the planet) and introduced into the carbon cycle. Fossil fuels give us energy that can be used in many ways, such as electrical generation in power plants, or transportation using motor vehicles. The burning of fossil fuels, however, introduces large amounts of carbon dioxide and other forms of carbon (such as methane and black carbon) into the atmosphere. These human induced carbon fluxes can be read about more here.
The input of this carbon from humans may not look like much compared to the tremendous amounts of carbon cycled naturally, however it introduces a net increase of carbon each year which is what matters. This net increase is responsible for worrying climate change problems such as global warming and ocean acidification. Figure 2 below shows exactly how much human carbon is input each year, and where it goes.