The set command is a SQL*Plus command.
It is an instruction telling SQL*Plus how to act.
For instance, you can check the setting of feedback by typing
SQL*Plus will respond with the following:
FEEDBACK ON for 25 or more rows
The width used to display numbers is changed by the set command. You check it by typing
SQL*Plus will reply as shown here:
To shrink the display by typing
set numwidth 5
Here, this means that all number columns will be five digits wide.