Bundles are groups of Heroes of the Storm content, often themed, that can be purchased with Gems. Bundles are Dynamic, meaning that if you already own some of the items sold in the bundle, the bundle's Gem price will be adjusted and lowered accordingly.
You'll find bundles centered around a particular event (e.g. the Echoes of Alterac event bundle), a particular hero, etc. Bundles typically offer Heroes and/or cosmetic items.