basically what they're saying is you can take aboard a wheelie bag and a handbag. It could be a rucksack as long as it fits the dimensions and the weight limit. Their planes aren't huge and don't have a lot of baggage space; and they don't want a lot of people checking luggage in for the hold, as loading and unloading takes time, makes them late and costs them more in airport charges.
Sometimes if there's a lack of space they have to put luggage in the hold even if it's the right size and weight, but in that case they won't charge you.