What does the 1701 mean on the enterprise?

Answer 1

Its its registry number, the enterprise is the 1701th Federation starship, later vessels named enterprise, like the Enterprise-D from The Next Generation have the same number as they are called the same so have the same registry number, just with a letter on the end.

Answer 2

NCC-1701 is the registry number of the Constitution-Class U.S.S. Enterprise that was in service from 2245 until 2285, first seen in the original series' original pilot, The Cage, under the command of Captain Christopher Pike and last seen (in the original time line) in the movie Star Trek III: The Search for Spock under the command of Captain James T. Kirk.

Every ship since then that bore the name U.S.S. Enterprise had a registry number that started with "NCC-1701-".

The ship with registry number NCC-1701-A, referred to as the "Enterprise-A", is also a Constitution-Class star ship. It was originally launched into service in 2286 under the command of Captain James T. Kirk, as seen in the movie Star Trek IV: The Voyage Home.

NCC-1701-B is the registry number of the Excelsior-Class U.S.S. Enterprise seen at the beginning of the movie Star Trek: Generations.

NCC-1701-C is the registry number of the Ambassador-Class U.S.S. Enterprise that was commanded by Captain Rachel Garrett until its destruction in 2344, as seen in the ST:TNG episode Yesterday's Enterprise.

NCC-1701-D is the registry number of the Galaxy-Class U.S.S. Enterprise that went into service in 2363 under the command of Captain Jean-Luc Picard, as seen throughout the TV series Star Trek: The Next Generation and last seen crashing in the movie Star Trek: Generations.

NCC-1701-E is the U.S.S. Enterprise that was first seen in the movie Star Trek: First Contact.