Well, the meaning is pretty simple. It goes back to a short story Clarke wrote in 1951, called, "The Sentinel." The plot described a mission to the Moon, in which an alien artifact is found, and when it is disturbed, generates a signal directed to an unknown destination. Here is the protagonist's own explanation, as he reflects on finding the artifact:
"When our world was half its present age, something from the stars swept through the Solar System, left this token of its passage, and went again upon its way. Until we destroyed it, that machine was still fulfilling the purpose of its builders; and as to that purpose, here is my guess.
Nearly a hundred thousand million stars are turning in the circle of the Milky Way, and long ago
other races on the worlds of other suns must have scaled and passed the heights that we have
reached. Think of such civilizations, far back in time against the fading afterglow of Creation,
masters of a universe so young that life as yet had come only to a handful of worlds. Theirs would have been a loneliness we cannot imagine, the loneliness of gods looking out across infinity and finding none to share their thoughts. They must have searched the star clusters as we have searched the planets. Everywhere there would be worlds, but they would be empty or peopled with crawling, mindless things. Such was our own Earth, the smoke of the great volcanoes still staining the skies, when that first ship of the peoples of the dawn came sliding in from the abyss beyond Pluto. It passed the frozen outer worlds, knowing that life could play no part in their destinies. It came to rest among the inner planets, warming themselves around the fire of the Sun and waiting for their stories to begin.
Those wanderers must have looked on Earth, circling safely in the narrow zone between fire and ice, and must have guessed that it was the favorite of the Sun’s children. Here, in the distant future, would be intelligence; but there were countless stars before them still, and they might never come this way again.
So they left a sentinel, one of millions they have scattered throughout the Universe, watching over all worlds with the promise of life. It was a beacon that down the ages has been patiently signaling the fact that no one had discovered it.
Perhaps you understand now why that crystal pyramid was set upon the Moon instead of on the Earth. Its builders were not concerned with races still struggling up from savagery. They would be interested in our civilization only if we proved our fitness to survive by crossing space and so escaping from the Earth, our cradle. That is the challenge that all intelligent races must meet, sooner or later. It is a double challenge, for it depends in turn upon the conquest of atomic energy and the last choice between life and death.
Once we had passed that crisis, it was only a matter of time before we found the pyramid and
forced it open. Now its signals have ceased, and those whose duty it is will be turning their minds upon Earth. Perhaps they wish to help our infant civilization. But they must be very, very old, and the old are often insanely jealous of the young.
I can never look now at the Milky Way without wondering from which of those banked clouds of stars the emissaries are coming. If you will pardon so commonplace a simile, we have set off the fire alarm and have nothing to do but to wait.
I do not think we will have to wait for long."
That's preserved in the movie (the signal beamed from the Moon, when the sunlight hits the unearthed slab in Tycho), though the movie adds the concept that the alien race had a role in moving our evolution along, via telepathic suggestions emanating from the slab in the "Dawn of Man" sequence. And it adds the concept then of the star gate.