Weather and climate models do not represent the atmosphere in a continuous manner, but at either discrete points or averages. This is the case in the horizontal (east-west and north-south), vertical and in time.
Horizontal
In the horizontal these the discrete points are arranged in a two dimensional grid and hence are referred to as grid points. The grid can be regular or irregular. An example of a regular latitude/longitude grid would be where the grid points are located every 1 degree of longitude, in the east-west direction and every 1 degree of latitude in the north-south direction. Each grid point is associated with an area that either surrounds the grid point or lies between the grid points. This area is referred to as the "grid box".
Vertical
In the vertical the discrete points are arranged in levels.
Time
Weather and climate models usually represent time at discrete points, with a spacing that is called the time step. The time step is usually between a few minutes and half an hour, depending on the model.