You probably use some of them but never thought of it.
An opinionated product is one that takes a stance on how it should be used. It's not a tool for everyone, but for a specific type of user who wants to accomplish a specific goal. Instead of targeting anyone to use it, it's designed so only certain users will want to use it in certain ways.
For example, rather than providing lots of options for how to do things and how the interface looks, an opinionated product will have its own preferred methods and styles. Rather than giving you multiple ways to accomplish something, an opinionated product will force you to do things its way. This makes life easier for users who don't need flexibility; they just need something that works well right out of the box without having to set up their own infrastructure or customize anything themselves.
I see non-opinionated products as restaurants that have different type of food on their list. It can suit you in some cases but it generally means that they don't have a single thing where they're excellent at. When you don't know what to choose always prefers a place where there's not much choice, it also means that it'll will be easier for you too choose.
Last year I tried Linear for the first time (they were still in Beta) and that's exactly the feeling I've got, Linear was already an opinionated product. The product follows a clear vision that you can feel when using the product. They have chosen a specific way to solve a problem and aims to deliver the best experience possible in that context. Back then, there weren't not much you couldn't do with the product but still you were able to feel a strong position on how to do things.