The sestertius is one of several Roman denominations. In an earlier post I wrote about the denarius. The sestertius was introduced at the same time as the denarius, in 211 BCE. Its name means "two and a half" because its initial value was the equivalent of 2.5 asses (an as was an existing copper coin). As it can be seen, its value was 1/4 of a denarius.
Initially, the sestertius was, like the denarius, a silver coin. However, after a reform by Augustus in 23 BCE, it was converted into a brass (copper and zinc) denomination. By then the denarius was valued at 16 asses, so the sestertius was valued at 4 asses.
The sestertius, just like the denarius underwent progressive debasement. By the 4th century CE it was not used anymore.
Other Posts about ancient Roman Currency