Usually, a child under 2 flies for free as he/she sits on the lap of one of his/her parents and therefore is not entitled to any luggage allowance.

"Infants under the age of two / on the date of their return flight" means if the child is still under two years old when they take the return flight. "No allowance" means "no luggage allowance". I think general luggage allowance is about 20-25kg for check-in luggage, and about 10-15kg cabin baggage, per person. Basically, this means that travelling with a child under two does not mean the parents can bring another 30-40kg of luggage onto the plane. The young child's luggage must be included in the parents' luggage.