Here is a simplified example of a task that I would like to perform on a
spreadsheet with several thousand entries/rows.
I do not know what the operation is called so don't know how to search for
an answer myself.

In my main spreadsheet the rightmost column contains a single category
number, from a range of several hundred.
The same category number can be used on many rows.
I wish to place a text description/label in the next cell on the right.
I have a 2 column table/spreadsheet with the category numbers in the first
column and the matching label in the corresponding cell of the second
column.

How do I apply the contents of this table to my main spreadsheet so that
the appropriate description/label will be incorporated into the new right
column?

Thanks for any pointers
Brian Tozer