Because of the intent to make sure some things are done or not done on certain days, there are various ways of measuring or defining when evening has arrived: sunset or the visibility of three stars in the night sky. So, if you want to make sure you do not do something on a certain day, like work on Shabbat, you should stop doing those things as the sun sets. But if you want to make sure the day has arrived, you should wait for three stars.
Erev also has a meaning that is less common these days. In this older meaning it refers to the time before the holiday has actually begun. During this time, one can still do the things that are forbidden or need to be done before the holiday in preparation. In this meaning, it can refer to the day before the holiday or event.
Erev Shabbat and the dinner that starts Shabbat are an important family gathering time for many Jews, even those that do not observe other religious rituals. Kol Nidre, the service on the evening of Yom Kippur, or Erev Yom Kippur, is probably the most widely attended of any Jewish religious service.