I read a shloka (source not known)" Punya shloko Nalo Raja Punya shloko Yudhishtirah. Punya shloko Videhascha Punya shloko Janardanah." What is meaning of Punyashloka in the above context
Puṇyaśloka (पुण्यश्लोक) means well spoke of good fame or about whom we should remember for his great actions, according to Sanskrit dictionary.
Definition: mf(ā-)n. "well spoken of", of good fame or reputation (puṇyaślokeyakarman keya-karman- mfn.one whose actions must be praised in auspicious verses)
Here Nalo Raja = Raja Nala, Yudhisthira, Videha = King Janaka, Janardana = Lord Krishna are whose great action for preserving Dharma and righteousness can be found and learned from Itihasa and Puranas. So, this shloka is for remembering them.
You can read English meaning of this Shloka from sanskritdocuments:
पुण्यश्लोको नलो राजा पुण्यश्लोको युधिष्ठिरः ।
पुण्यश्लोको विदेहश्च पुण्यश्लोको जनार्दनः ॥ १६॥
Meaning:-Raja Nala, the truthful Yudhisthira, Janaka the king of Videha, and Janardana (i. e.) Lord Krishna -the very utterance of these names sanctifies the tongue.