The phrase "significant dent" means a noticeable or considerable impact or damage. It can be used to describe a major reduction or impact on something. Full definition
This may seem counterproductive at first, but taking on a new credit card is actually one of the easiest ways to make a really significant dent in your debt. (creditdonkey.com)
I didn't even need to drop the phone myself to figure this out: someone who previously used my G6 review unit quite clearly dropped the G6, leaving a few significant dents around the edges. (techspot.com)
Because pricing is an important driver of profitability, getting it wrong can cause a significant dent in your bottom line, and it can make the difference between remaining profitable or going out of business. (smallbusinessesdoitbetter.com)