"Oh God I can see forever!" is an image macro series featuring a subject that is usually shocked or stunned, with the phrase added as the caption.
It is implied that the subject is peering into an endless void, has just seen something no mortal should ever see, or something so horrifying that he/she will never be the same again.
On a smaller scale, the phrase could be used outside macros to describe a very powerful experience, mostly shocking or life-changing.
While the idea of "I can see forever" was perhaps inspired by the central message of H.P. Lovecraft's "The Call of Cthulhu," the phrase itself does not appear in the story. It seems that many on the internet conflated this quote with H.P. Lovecraft because of a picture of a photoshopped Family Circus.
On Google, the phrase "I can see forever" had been entered at least 235,000 times, and "Oh God I can see forever" at least 1,100,000 times.