There was actually no significance to the shooting star in the movie. It was just a happy accident that was captured on film according to the 1995 documentary The Making of Steven Spielberg's Jaws. This site explains it in more detail:
Ironically, Spielberg’s first cinematic shooting star occurred
accidentally rather than by design. In the final act of Jaws, a
shooting star can be seen during one of the night sequences, when
Brody is loading his gun. However, this wasn’t a special effect.
According to the 1995 documentary ‘The Making of Steven Spielberg’s
'Jaws’’, the shooting star occurred naturally, and Spielberg and his
crew just happened to be in the right place at the right time to
Apparently, this happy accident turned into somewhat of a theme in future Spielberg movies involving shooting stars:
A happy accident it may have been, but it sparked off a reoccurring
theme. Along with Close Encounters and Jaws, shooting stars can also
be seen in Indiana Jones and the Temple of Doom and The Adventures of
Tintin: The Secret of the Unicorn. Some have also suggested they’re
visible in Hook and Saving Private Ryan, but so far I haven’t been
able to find any clear-cut evidence for either.