Show media players embeds a media player in your
result page for every matching QSO, which makes it
easy to playback the audio for that QSO. But every media player in the
resulting page also immediately
downloads the corresponding QSO audio, which can strain your internet
connection for a while.
Search by RE means the "Callsign" you enter is a
UNIX regular expression.
Have a look at
the documention
for details.
For a simple example, this string will return all the QSOs in the log:
~.*~