The VLOOKUP function is important because it allows you to use Excel as a database, and not just a spreadsheet. The distinction is that a spreadsheet is a single two-dimensional table, whereas a database is a series of two-dimensional tables that are linked together. Without the VLOOKUP function, the featured spreadsheet would require an extra item cost column in the sales journal which would need to be manually entered and, in practice, could stretch to thousands of rows.
The simplest and most commonly used form VLOOKUP has three parameters; the entry you want to lookup, the table in which it will be found, and where in that table it will be found. It is entered as "=VLOOKUP(Entry, TableRange, TableColumn)". TableColumn can be confusing. Excel will always look for the Entry in the first column of the table. This is implicit in the use of the function. Therefore, TableColumn should be the number of the column containing the output data. In the video, the price is in the second column, where the item is in the first, so TableColumn equals 2.
Many people consider knowledge of the VLOOKUP function to be a distinguishing feature between beginner and intermediate Excel use. As you may be producing spreadsheets for people who cannot write the function, it is important your sheets accessible. It is worth using range names to define lookup tables and, on occasion, placing them out of sight on separate sheets.