Complex sentence: Baal teshuva movement ("return (to Judaism) movement") refers to a worldwide phenomenon among the Jewish people that began during the mid-twentieth century whereby large numbers of previously highly-assimilated Jews chose to move in the direction of practicing Judaism.
Split into simpler sentences: Baal teshuva movement ("return (to Judaism) movement") refers to a worldwide phenomenon among the Jewish people. It began during the mid-twentieth century, when large numbers of previously highly assimilated Jews chose to move in the direction of practicing Judaism.