I was reading Modern Chess Openings by Nick de Firmian, and found "columns 1-24 deals with 2...exf4 " in relation to a theory table. So what does "columns 1-24" mean?
This table can be thought of as a tabular representation of an opening tree of in-theory moves where the rows (y-axis) represent the flow of the game and the columns (x-axis) refer to variations/branches in the tree that can occur.
The author in your quote indicated that 2...exf4 spans a family 24 variations/sub-variations that are noteworthy (and considered theory) for reference.
Here's an example for another such page on these NCO/MCO opening references.
If this is confusing, let's consider a simpler example with the first few moves (with some early deviations) from the popular Danish Gambit.
Here's what a fragment of the opening tree would look like.
If I wanted to represent/transform this tree into a table, it would look like this.