Written by George Harrison as a tribute to his then-wife, this was considered by John Lennon himself to be the best song on the Abbey Road album. The 1970 promotional film that followed (and is today's post) was recorded after John Lennon announced his departure from The Beatles - the four members are filmed at their homes with their respective wifes and edited together.
The song itself is on its surface a straightforward love song - the narrator is complementary towards the source of his affection. However, the bridge brings the straightforward love to reality - that there is a lot of things unknown for the future, but lovers will still strive together towards an unknown goal. The guitar solo after that bridge by Harrison is considered one of the greatest of all time.