An "entry stamp" is a mark or imprint given by immigration officials on your passport when you enter a country. It confirms that you have arrived and specifies the date and place of entry. Full definition
The passport must be valid for at least 6 months beyond your date of entry and must have at least 1 blank page for entry stamp. (caradonna.com)
One page is required in your passport for entry stamp. (caradonna.com)
Additionally, since Prague is in the European Union's Schengen Area, which allows passport - free travel between countries, he could theoretically have gotten an initial entry stamp from any EU country, not just the Czech Republic. (vox.com)