Google is a company that makes a large amount of the internet, but their name is synonymous with their flagship product, which is the most dominant search engine there is. When you type something into Google, they try to find whatever that is and show it to you. Because having people go to your website is the key to making money on the internet, there is an entire job (called "search engine optimization") dedicated to having a specific website show up high in these rankings.
As such, Google has a large amount of control over what people see or think when they look up any given thing. This power might be fine to leave to them when the thing you're looking up is not super important, but once you consider its implications in (for example) political elections, it's not too hard to realize that concentrating this amount of discursive control in the hands of a single for-profit organization makes it less likely that they are always doing what they say they are doing.
In addition, it's common knowledge that Google cooperates with various world powers on their surveillance and propaganda projects—notably, the United States and China. So, given specific and concrete examples of their willingness to do something other than deliver the "most relevant" search results, we can imagine that they are providing less-than-faithful results or information in a wide variety of cases.
Google does more than just provide a search engine, though—they also make phones, stereos, and an extensive range of other tools and services relating to (and sometimes not at all relating to) the internet. Many of these products and services are capable of collecting and sharing information about what happens near them or what they do. Considering the aforementioned willingness to cooperate with governments repressive and less-repressive alike, these capabilities add a healthy dose of the spice of danger to Google's position overall.
In the main, Google's search results are what they're supposed to be (even if they're easy to manipulate). But Google is also a company that does a whole lot of things, and they have a lot of largely unsupervised power that they can use to do whatever they want. Not all of it is great. We have to call Google fairly dangerous.
