Starting Shabbat early during the summer

Question

Is there an earliest time one may begin Shabbat during the summer months?

We were traveling south and stopped at a Jewish community for Shabbat. The Shul there davened Mincha friday at 6:10, then added an extra prayer between Mincha and Kaaballat Shabbat to add 3 or 4 minutes. The Rabbi's explanations was they needed to add the extra time so as too not daven Maariv before the "earliest allowed time". I never heard of that before and was confused since it was still daylight out while walking back after davening was over.

Answer 1

The earliest time that Shabbat may be accepted is called "Plag ha-Mincha", which occurs one and one quarter 'hours' before sunset. An 'hour' for this purpose, is defined as 1/12 of the day (from sunrise to sunset).

Answer 2

Addenda to ADDeRabbi's answer:

Here's a good writeup on the topic of starting Shabbat early.

Here's a MyZmanim link for this past Friday in a random Southern US city, showing that Plag was 6:23 pm, which fits with your story.