Waht does the "cdft 1 2 charge 1.0" mean?

In the documentation of NWchem 5.1.1 p128, there is a constrained DFT example as:

geometry
symmetry
C ..
O ..
C ..
O ..

basis
..
end

dft
...
cdft 1 2 charge 1.0
end
task dft

What does the colored line mean?