"Turnover" refers to the rate at which employees come and go. A place with a high turnover is constantly replacing employees who leave. In this case, the author is saying that it is difficult to get to "know" the people working at a store since the turnover is so high.
Instead of seeing "Bob" or "Sue" every morning at the local store, who knows which pack of smokes you want and if you want cream for your coffee, there is a constant stream of people who don't know you and don't care.
Student or Learner