Personal Item is a total optional thing but comes in hand sometimes. Airlines also allow you to carry one personal item (like a small bag or backpack). This item should fit underneath the seat in front of you, literally removing your feet extra space. It is recommended to have a foldable backpack/bag for emergencies (like when you come back home and bought too much or for when airlines force you to check your carry-on due to space limitations) rather than having an actually small suitcase.
You can browse some examples here: Foldable Backpacks on Amazon