An heirloom is something passed down in a family for generations. Your grandma's prized necklace could be an heirloom.
If you know that an heir is a younger person in a family who will inherit things when relatives die, you have a clue to the meaning of heirloom. An heirloom is a special thing that's handed down, sometimes through a will, but often just from person to person. A portrait of an ancestor could be an heirloom. Many families pass down heirloom jewelry. If you hear someone say, "This belonged to my great-great-grandfather" they're probably talking about an heirloom.