An Ephemeris gives the positions of celestial bodies in the sky at a given time.
The information that the Ephemeris Calculator provides is usually found in an Ephemeris table. Most astrologers use the tables that Astrodienst provides for every year. For an example, check out Astrodienst's 2018 tables.
Jessica Adams has a great post explaining how to read an Aphemeris table.
The tables make it easy to look at the planet's positions for a given day.
The Ephemeris Calculator is designed to make it easy to do the opposite lookup, and find the dates when a planet will be in a specific position. This is commonly done when looking for conjunctions with a person's natal chart.