Dustproof is a feature similar to waterproof. Waterproof phones can protect themselves from water; similarly, dustproof phones can protect themselves from dust and dirt. It is also waterproof like inexpensive budget phones.
Oppo Reno8 Lite comes with an IP68 rating which proves that the phone is water resistant as well as dust resistant. Apart from protecting yourself from water, it can also protect your phone from dust. So this is a special attraction for a phone.