MODFLOW is a finite-difference groundwater flow modeling program, written by the United States Geological Survey (USGS), which allows you to develop a numerical representation (i.e. a groundwater model) of the hydrogeologic environment at a field site that you are investigating.

It uses the finite-difference method to divide the groundwater flow model domain into a series of rows, columns and layers, which defines a unique set of grid blocks (i.e. model cells) to represent the distribution of hydrogeologic properties and hydrologic boundaries within the model domain.

When you develop a groundwater model, you assign properties and boundaries to these model cells, and MODFLOW uses the cell dimensions, property values and boundary values to write a set of finite-difference equations that it solves to calculate the hydraulic head at the center of each model cell.

More here: https://www.waterloohydrogeologic.com/2012/06/22/what-is-modflow/