A string is just a collection of characters, in programmer speak. When you use quotation marks in your search to find exactly what you want. If your search was:

dog “fast drive”

Google used to show results that only had both the word “dog” and the joined phrase “fast drive” in the same result.

Now it feels like Google uses that as a suggestion, giving you “dog” and any combination of “fast drive”, “fast driver”, “fast driving”, or whatever else Google thinks you want, instead of what you asked for. Or if they don’t find it, they serve you up whatever they want, with a small message about there being no matching results.