And just incase you were asking more on what Catalogging is... It's when you a person drops an item, you pick it up, and then drop it back down. Similar to how you can trade a Pokemon back and forth for Pokedex data, picking up an item causes the item to appear in your catalog in the Nookling shop and let's you reorder from it (for the most part--some items are not reorderable*)
*And then you may ask, why would you want to catalog it? There is a badge for Catalog Completion so... And if it's an item you don't really care for, but would like to have in your Catalog for Completion sake or the badge, it's usually a cheaper alternative.