The plot armour is not the armour at all. People use it all the time as it was something better not to have.
As with any words on the Internet, it became a buzzword which lost meaning as it is used all the time outside context, but I'll try anyway:
Plot armour is everything that needs to happen in order for the plot to exist.
Without the plot armour, there is no plot at all.
Of course, the protagonist is the most "plot armoured" character as without the protagonist the story wouldn't exist. A side characters need to die for the protagonist to mourn or have enemies to defeat, but the protagonist, he is indispensable. Even in tragedies, he can't die until the story esop kicks in, because plot (the narrative) needs him, only to needing him die moments after. Because that's the plot, the narrative, the message. Everything the plot needs has the plot armour. Everything in the story needs to die, lose it because it needs to die.
Without Saitama, there is no plot of One Punch Man (and no joke). Without plot armour, there is no Saitama.