What does nitrogen purging mean?

Nitrogen purging means to inert the atmosphere of hydrocarbon vessels and lines. In other words to limit the oxygen contents so that it cannot react with the hydrocarbons and make some explosive mixtures.

Nitrogen Purging in Gas Well: (Oil & Gas Industry Terms, Well Activation)

Generally for gas well activation air compressors are used. Air compressor puts in oxygen into the Hydrocarbon system in the well, this results in explosive mixture (as hydrocarbon & oxygen can explode by a single spark). To reduce oxygen content in the well, Nitrogen is circulated at high pressure @ 80-100 Kg/cm2 from annulus (casing) and returns of gases are taken out from tubing of the well.

The Nitrogen purging job brings down chances of explosion / explosive mixtures.