Commercial substance refers to when a business exchanges one product for a completely different product which changes the expected future cash flows from the transaction. Basically if the financial position of a company were to change, the exchange would be considered a non monetary exchange that contains "commercial substance". According to IFRS, a non monetary exchange that does have commercial substance is referred to as an exchange of "dissimilar assets".
Example:
Adequate Disclosure, Inc. exchanges a car for a building with Inadequate Disclosure, Inc. This transaction contains commercial substance, since the output from the car will yield $5,000 more revenue than the revenue yielded from the building.