Cabin baggage is also known as carry-on or hand luggage, and can be a backpack or a small suitcase. Cabin baggage is usually included for free, but this is not a rule. Some carriers do not offer it at all or they will charge you to add it.
It’s best to keep your valuables in your cabin baggage. There are limitations on the amount of liquids you can carry as well as prohibited items which cannot enter the cabin. You’ll take cabin baggage through security, and keep it with you once on the plane— either under the seat in front of you or in the overhead compartment.
Some airlines or itineraries offer free cabin baggage, but it’s possible you’ll need to pay extra to add it to your itinerary.