What is Tosefet Shabbat (adding to Shabbat)?

Tosefet Shabbat (adding to Shabbat) is a mitzvah to accept the sanctity of Shabbat a few minutes before it officially begins at sunset, and to extend it a few minutes after it officially ends. By voluntarily beginning Shabbat earlier, we demonstrate our love for the day, we are so eager for its holiness that we bring it in before it's strictly required. This concept teaches that holiness can be expanded through our own initiative.