When a hostel advertises free breakfast it can mean many things.
Usually breakfast in a hostel consists of bagels coffee and occasionally some donuts or pastries. Sometimes there will be yogurt sometimes fruit. Occasionally there will be raw eggs for you to prepare as you please. Some places will have a vat of pancake mix for you to use (if you can find a frying pan and fight your way to the stove).
Once there was cereal, fruit, yoghurt, bread for toasting, bagels & jam, pastries, fruit juice, tea, coffee and hot chocolate. That was breakfast to remember. I look back fondly on that brekkie. It was great. I pine for that breakfast as I sit nursing my bowl of porridge in the kitchen of the hostel that does not include breakfast in the price of the room.