It’s a mod source.
Basically it’s just a constant value, whereas a LFO for instance cycles from lower to higher values if, for example, it was set as the source triggering Pan, creating a sweeping from one speaker to the other, DC being a constant value just moves it toward to that speaker (when used as source for destination Pan in the mod matrix).
Think of DC as DC electricity and a LFO as AC electricity. Constant value vs changing value.