According to my Webster's
DEFINE show-stopper
DEFINITION 0
show-stop-per \-,sta^:p-er\ n (1946)
1 : an act, song, or performer that wins applause so prolonged as
to interrupt a performance
2 : something or someone exceptionally arresting or attractive
<the gold crown was showstopper of the exhibition>
Whether this definition qualifies as 'plain' English is up to the
reader...
Martin