Commercial Use means you can use the file you've downloaded in ways that result in commerce (aka someone is making money).
For example, use a file to create marketing collateral (brochure), advertising pieces (Facebook ad), and products you sell (t-shirts)*
*If using a file for merchandising purposes (mugs, t-shirts, bags, etc.), you must alter the file to add value to the design.
For example, you can't just take this crab and put it on a t-shirt:
You can however, take that crab and make a fun pattern to print on a t-shirt like this: