In Guns, Germs, and Steel, these terms are only used in Chapter 1. They refer to a type of spearpoints and the people who made them. As Diamond says on p. 45 in the paperback edition,
The oldest unquestioned human remains … in the United States south of the Canadian border… are called Clovis sites.
Clovis people, then, were (at the time that Diamond wrote this book) the people who were believed to have colonized the Americas. Before they came sometime a little bit before 11,000 BC, there were believed to be no people in the Americas.
How, then, can there be “pre-Clovis” sites? These are sites that some people believe show that there were humans in the Americas before the Clovis people arrived. At the time that Diamond wrote this book, most scientists did not believe that the pre-Clovis sites were truly legitimate. Now, there are many more people who believe that there are real pre-Clovis sites. At any rate, pre-Clovis refers to people who may or may not have been in the Americas before the Clovis people.
Post-Clovis, of course, refers to the situation after the arrival of the Clovis people. Diamond only uses this term once in the book, on p. 48. There, he talks about the possibility that charcoal from post-Clovis peoples filtered down into soil that is below the Clovis layer, thus making it look like there were people in the Americas pre-Clovis.