The meaning of setting columns to 2:2 is described in the documentation of CombiTable1D. Just open the CombiTable1D class and hit F1 to see the documentation.
First, 2:2 is the same as the one element array {2}. The interpretation is that the second column of the data matrix will be used for interpolation of the first (and in this case the only) output.
If you think of the interpolation in terms of a function plot, with function input values on the x axis and function output values on the y axis, then the x coordinates of the data points are always given by the first column in the data matrix, and the y coordinates by the columns specified using the columns parameter. Each column of y coordinates defines an interpolation, and the block will have a corresponding input/output pair for interpolation using this column. Hence, in your case, where the block just has a single input/output pair, the columns parameter should only specify one column, for instance {2} (or 2:2).
Of course, with a data matrix having only two columns and a block with just one input/output pair, specifying the second column is the only sensible thing to do, and this is also what you get by default. (You will notice the grayed out setting 2:size(table, 2) in the field for columns if you don't type anything.)
