system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Dr. Who. Basic premise / History / Popularity and where to begin if one has never watched it.
gpt
The basic premise is that the Doctor is an alien (species: Time Lord) who travels around in a spaceship/time-machine called the "TARDIS."  The TARDIS is bigger on the inside and has a camouflage circuit that's been jammed to "London Police Box, 1950s" since the first episode.  He tends to be accompanied by "companions," people who travel with the Doctor on his adventures.  

The show began in 1963 and, thus far, 12 men have played the Doctor.  This is explained in-universe by Time Lords growing a new body when they die.  The show went off the air in 1989.  There was an attempt to revive it in 1996 with an American-made television movie, but that didn't get the green-light for a full series.  It was successfully revived in 2005 with the Ninth Doctor, played by Christopher Eccleston.  

It was a cult show in Britain for many years, aimed mostly at younger audiences but having a strong following among science fiction fans, but the revival has brought it a more mainstream popularity.  

I'd recommend getting started with Eccleston's episodes--Series 1, which aired in 2005.  Watch the modern series, then go back to the older stuff.  There's no need to slog through every episode (which is impossible anyway, given that a good many were destroyed in the old days of non-ubiquitous recording), just pick up a story or two (stories are several episodes long, though) from each Doctor.  And give the TV movie a look--it's not perfect, but the 8th Doctor's acting is very good.  Eric Roberts as "The Master"...your mileage may vary.