Water-resistant hearing aids can withstand some exposure to water. Simply put, they provide a certain degree of protection against sweat, light rain, and occasional splashes. However, they're not considered "waterproof," so you don't want to submerge them into water for extended periods, such as swimming or bathing.
In contrast, you might think that waterproof hearing aids offer more protection. The issue is, however, that waterproof hearing aids don't exist—at least not yet—meaning nothing on the market provides 100% protection against water damage.