OpenFOAM guide/Implicit equation
Implicit equations are those where the dependent variables rely on the values of one another from the current time. In other words, all dependent variables must be solved simultaneously, which is only possible using an iterative method.
OpenFOAM's fvm namespace defines implicit operations.
Implicit methods have a less stringent timestep size restriction than explicit methods. Therefore they are preferable for steady-state problems, since a steady state problem can be viewed as a transient problem with an infinite timestep.