The point of it…
This function brings up a calculator, where you can choose functions to calculate values which interest you. For example, a word list routinely provides the frequency of each type, and that frequency as a percentage of the overall text tokens. You might want to insert a further column showing the frequency as a percentage of the number of word types, or a column showing the frequency as a percentage of the number of text files from which the word list was created.
This wordlist has a column which computes the cumulative scores (running total of the % column).
How to do it
Just press Compute  New Column and create your own formula. You'll see standard calculator buttons with the numbers 0 to 9, decimal point, brackets, 4 basic functions. To the right there's a list of standard mathematical functions to use (pi, square root etc.): to access these, doubleclick on them. Below that you will see access to your own data in the current list, listing any numberbased columnheadings. You can drag or doubleclick them too.
Absolute and Relative
Your own data can be accessed in two ways. A relative access (the default) means that as in a spreadsheet you want the new column to access data from another column but in the same row. Absolute access means accessing a fixed column and row.
Examples
you type

Result  for each row in your data, the new column will contain:

Rel(2) ÷ 5

the data from column 2 of the same row, divided by 5

RelC(2)

the data from column 2 of the same row, added to a running total

Rel(3) + (Rel(2) ÷ 5)

the data from column 2 of the same row, divided by 5, added to the data from column 3 of the same row

Abs(2;1) ÷ 5

the data from column 2 of row 1, divided by 5. (This example is just to illustrate; it would be silly as it would give the exact same result in every row.)

Rel(2) ÷ Abs(2;1) × 100

the data from column 2 of the same row divided by column 2 of row 1 and multiplied by 100. This would give column 3 as a percentage of the top result in column 2. For the first row it'd give 100%, but as the frequencies declined so would their percentage of the most frequent item.

You can format (or even delete) any variables computed in this way: see layout.

See also: count data frequencies, column totals, colour categories, add a Set column