You earn 100,000 in a year but you find various ways to report less than that, so you only pay tax on for example 20,000. There are many different ways to do that, such as unreported sales (often cash), VAT fraud, issuing nonsense loans, and just good old fashioned cooking the books. Sometimes it's enough to just place money overseas and there's just no one at the tax authority with the time to go through all the reports.
Lack of enforcement means no one is looking at all︀ the available data and connecting the dots, either wilfully or due to incompetence/lack of resources.︁ Most of the tax fraud going on in Greece wouldn't be possible in countries with︂ more efficient and more effective tax authorities. And Greece is catching up.