It means a secret service agent will feel you up with a metal detector to ensure that you're not armed. (I enjoyed the style and pacing of this article: fairly entertaining.)
You'd likely have to leave or face a felony with 10 years prison sentence. (Per H.R. 347, it's 10 years for entering a secret service permanent or temporary restricted area without permission with a firearm.)
So, if I was there before anybody who had anything to do with the President showed up, and I had a legal firearm on me, and SS shows up and decides that I'm suddenly in a restricted area, I'm immediately a felon?
A couple of things: first, where the commenter you're replying to said "leave or face a felony" (emphasis added), and, second, where you happen to know that the author lives in DC, where there is no "had a legal firearm on me" unless he happens to be a law enforcement agent himself, which seems not to be the case.
And I have no particular desire to be armed near the President, but I may have a desire to be armed in public without being shooed away all of a sudden.