Oracle allows a value of "null" for entries into the tables. This could mean a missing observation, for example. It doesn't mean a zero entry - a null wind speed and a zero wind speed are very different.
If you select an entry in the allow-null column of the form, then values of that column which are null will be allowed to be printed. This is not a lot of use if you only select one table element to be printed out, but if you select several elements you may be happy to have one of them null.
If you don't select the entry in allow-null, then sql for "and entry-name is not null" will be added to the query and a null in that entry will prevent that row from being selected for output. This is useful for items such as latitude or longitude in a ship report, which the entire row is useless without.
The default is all items unselected, ie none allowed to be null.