Something that's heavy weighs a lot, either physically or emotionally. It's hard to lift a heavy backpack, and it's hard to handle a heavy topic, like the meaning of life.
Bulky, substantial things are heavy, and you can also describe people as heavy, especially if they're big-boned and carry a lot of weight. Thick or strong things are sometimes called heavy as well: "The apple tree was heavy with blossoms, but when the heavy wind started up, they all blew away." Heavy breathing is fast and labored, and heavy food is just too rich or filling.