The formula belongs to a table. When you see @, it means "look at the value in the same row where the formula is, and pick the value under the field between brackets.
There is a table named Table_Inventory_Items The formula says: If the value under the field (column) [item_2_category_id] in the same row of the cell containing the formula is "" (empty), then produce "" (empty), otherwise, If that same value under [item_2_category_id] is equal to the value in cell A5 of sheet 1, then produce "" (empty) as well, otherwise look for the value in column [item_2_category_id]] in the first column of another table called Table_Inventory_Item_Categories and produce the corresponding value in column 2 of that table. I hope this helps.
I recommend you read or watch videos about Excel Tables. Knowing Excel Tables and taking advantage of them, allows you to step up your game in Excel.