Most newsgroups are unmoderated. Though most people do post messages that are on topic, some do not. So you never know what may be posted.
Parental supervision is advised.
Think of Newsgroups as a gigantic post office with thousands and thousands of mail boxes. Each mail box contains mail on different topics. Let's say you're interested in Siamese cats. You look in the mail box about this subject and you can read mail that other people leave. You can also post mail to either the originator of a mail message or to the mail box for all to read.