This is the 9583rd most frequent Japanese word.
罰
Punishment.
Here, '罰する' is the verb form meaning 'to punish,' derived from the noun '罰.'
悪いことをしたら罰するべきだ。
If someone does something bad, they should be punished.
In this context, '罰' again is used as a noun to specify the nature of the punishment.
彼の罰は社会奉仕活動だった。
His punishment was community service.
In this sentence, '罰' is used as a noun meaning 'punishment' to denote penalties imposed on someone for a wrongdoing.