For the computer to make sense of it, the normally continuous spatial dimension "x" must be discretized. This is what this node does. In essence it represents the boundaries and the accuracy of the simulation. This node is the basis of all spatial simulations done in Composer.
Spatial Dimension contains 3 input fields that are used to generate the discrete spatial dimension. The upper inputs represent the lower and upper boundaries of the spatial dimension, while the last input gives the number of points in the generated space.