A void filler is the additional material that that's used to fill the empty space inside your packaging. Void fillers help keep your product in place, prevent it from being shifted around during transit, and also protects your products from breaking.
Common Types of Void Fillers:
Consider also using custom box inserts to keep your products in place when shipping. To choose the right type of void filler for your packaging, you'll need to consider how well it cushions your products, how much padding/protection your product needs, and the weight of your product.