Hello Wilna,
The Job Name field on the Work Order screen is meant to be a short text description of the job. Examples of possible Job Names are 'Service Call' or 'Computer Repair' or 'Tree Trimming' etc. It does not appear on the invoice.
The field as it appears on the table is called 'Comments' and can be found on the Activity table.
A hint for you going forward, you can check the table a screen is based off of and the name of the field; by editing the screen and right clicking on the design area and selecting show summary. ;)