Student or Learner
That's very observant of you, son. That's right, it's a bucket. And that bucket contains cement. They're bringing it up to the top of that building using a rope and a pulley because it's easier that way. They don't have to carry the bucket up and down the stairs. It'll be even harder if they haven't built the stairs yet. They just tie that bucket to the rope and just pull it up. They could also lower that bucket using the same rope. It's more efficient that way.
They're using the cement to build the building. Those people? They're called construction workers.
Yes, it's a very tall building. But it's not yet finished. They're still building it. See, there are still a lot of holes on it. (Actually, I'm thinking it could have been better phrased as See, the walls haven't been built yet. Those are just the posts and beams.)
Any way to explain the above better?
Not a teacher.
Using a bucket to lift supplies instead of trying to carry things up stairs/ladders is not just "efficient," it is the safe way to do it.