The summary area is a blank canvas where you can start making notes about the applicant, company or contact.
There are already areas in itris where you can skill code a person, or note down their job title or what they recruit for, so you can perhaps use this area to write about the less tangible aspects of that person.
On an applicant record, for example, you may want to record what the person was like, how they interview, what they do in their spare time, their likes and dislikes, etc. You may wish to copy in the summary from their LinkedIn profile. This will help you build a fuller picture of that person, whether its to build rapport with them or promote them for a role.
On a company record, you might want to record information about the Company based on their 'about us' section on their website.
It doesn't have to be one long text stream, you will see that you have lots of formatting options too.
This is also a searchable area, so if you later remember you made a particular note about somebody, but just can't remember who, you can run an advanced (Boolean) search of the summary area for each record type.