title: What does parameter key mean in dataframe.to_hdf()
body: <pre><code>DataFrame.to_hdf(path_or_buf, key, **kwargs)
  </code></pre>
  
  <p>In pandas official document, it is said that key is identifier for the group in the store.<br>
  But what does that mean? Still, I cannot find sufficient examples for that. I have tried some arbitrary values for parameter key, but I didn't see any difference between them. Sometimes, the api reference can be quite ambiguous. Can anyone offer me some examples to help me to have a better understanding of parameter key?</p>