Jonas learns of love from the Giver in Chapter 16 of The Giver. As with all that Jonas learns of life outside the community, he learns about love when the Giver transmits to him a memory. When Jonas is experiencing the Giver's memory, he does not even have a name for the emotion he feels.
The memory that he is given is a scene of a family at Christmas. There are parents, children, and grandparents. This kind of family scene is alien to Jonas, since there are no blood relationships between parents and children. There are reproducers in the community, and their offspring are assigned to various parents. The concept of a grandparent is one that Jonas is not familiar with because anyone old enough to be a grandparent, who is still productive, has been sent off to live with other childless adults, and there is no continuing relationship. Any unproductive adults are released. There is a beautiful Christmas tree in the memory, and the family is gathered around. Jonas gets a sense of light and warmth, as the people open gifts, which is also an alien idea in the community, and he feels an emotion he cannot identify, but that he likes very much, He asks the Giver what this feeling is and the Giver tells him it is love. Jonas then goes home and asks his parents if they love him. Being good members of the community, they reject his question as meaningless, love being a useless concept to them. Instead, they tell him they take pride in his accomplishments and enjoy him. Now that Jonas has felt love, he realizes that what his parents and the community have to offer him is not enough, and it is at this point that he really begins to see what an impoverished world he lives in.