Verizon Selects will target ads based on your mobile data usage
To be fair, Verizon has always contended that it doesn't share your data with anyone, but that doesn't mean Verizon won't sell it, and use it to create targeted ads which it will sell at a premium. The ads will be based on both your web browsing and app data to offer you coupons or promotions based on things you like.
Luckily, Verizon Selects is an opt-in service, unlike the data collection which you can opt-out of here (if you are still in the 30-day limit, which is unlikely if you aren't a new customer). So, if you don't want targeted ads, you don't have to have them. There doesn't really seem to be much benefit in opting in since your Verizon bill will stay the same, but you can join up if seeing more relevant ads is important to you.