This is discussed (at length) in the accompanying book. In short, the second monolith was an alarm device intended to show that the apes on the planet below had evolved to the point that they were able to reach their nearest planetary neighbour and to perform basic survey tasks. That being the case, it would now be of benefit for the monolith-builders to activate phase II of their plans, creating a second sun in the solar system to allow colonisation of the moons of Jupiter.
"Piecing things together after the event, we decided that the monolith
was some kind of Sun-powered, or at least Sun-triggered, signaling
device. The fact that it emitted its pulse immediately after sunrise,
when it was exposed to daylight for the first time in three million
years, could hardly be a coincidence.
"Yet the thing had been deliberately buried - there's no doubt about that. An excavation thirty feet deep had been made, the block
had been placed at the bottom of it, and the hole carefully filled.
"You may wonder how we discovered it in the first place. Well, the object was easy - suspiciously easy - to find. It had a powerful
magnetic field, so that it stood out like a sore thumb as soon as we
started to conduct low-level orbital surveys.
"But why bury a Sun-powered device thirty feet underground? We've examined dozens of theories, though we realize that it may be
completely impossible to understand the motives of creatures three
million years in advance of us.
"The favorite theory is the simplest, and the most logical. It is also the most disturbing.
"You hide a Sun-powered device in darkness - only if you want to know when it is brought out into the light. In other words, the
monolith may be some kind of alarm. And we have triggered it.