override ?

[Freeman Gilmore]

Would someone explain what line 5 does and what  "::"means?   Why line 6 is not enough.

1 \version "2.19.10"
2 sesquisharp = \markup { \sesquisharp }
3
4 \relative {  
5    \once \override Accidental.stencil = #ly:text-interface::print
6    \once \override Accidental.text = #sesquisharp
7    cis''4 }

Thank you, ƒg