When Vespasian became the new Roman Emperor in 69 AD, he wanted everyone to know that he cared about the people and was going to take care of them and not live luxuriously as Nero had. He tore down a lot of Nero's Golden House and made the land into a public park.
Vespasian also used his share of the gold from the looting after the First Jewish Revolt to pay for the construction of a new amphitheater where the Golden House had been. Because Vespasian's architects used the new method of building in concrete, he was able to build quickly and cheaply. We call this amphitheater the Colosseum, after the giant statue of Nero that stood near it. But its ancient name was the Flavian Amphitheater. Building the amphitheater made Vespasian very popular in Rome.
Colosseum interior
The Colosseum was a place where a lot of people could sit and watch entertainment. The entertainment was mostly people killing animals, or people killing each other. It was almost exactly like a football stadium today. It was built of concrete and marble and limestone.
Colosseum interior
The reason it looks so terrible in this picture is that a lot of the seats were made of marble and people have stolen them away over the years and burned them in lime kilns to make mortar and cement. The floor has also been taken away, so you can see the rooms in the basement where the Romans kept the animals and the equipment and stuff.