"Commercial use" defines in how many commercial settings the buyer can sell any resulting work. Typical cases of commercial use might be albums, mixtapes, iTunes sales, SoundClick sales, shows, and so on.
If the seller for example leases 2 commercial uses, the buyer can use this to sell the audio on his/her album AND on iTunes. This maxes out the 2 uses and he/she can't sell this song on any other platforms or settings.
"Unlimited commercial use" means the buyer can sell the resulting work in any way he/she wants as long as no other restriction such as max distribution or explicit sync lisensing applies.