Concentrate your forces in the queenside to gain control of it and go for space adventage there and capture strategic points in the position. Thus you can attack the king from different directions.
What do you mean by attacking queenside

I had no idea what this meant either until I really started watching Chessexplained on youtube.
It doesn't mean you are playing directly for the king. Maybe you will promote a pawn first, or find lines of defense for your king.
A great example of this is the Anand vs Carlsen Nimzo indian game.

I do understand king side attacking but what is a queenside attack
That's a very good question and something not discussed as much as it should be. Boris Shashin wrote a great and little regarded book called the "The Queenside Attack".
If the king castles on queenside - then same ideas as kingside attack.
Otherwise a queenside attack seeks three things IMO:
1/ Attack the king via the "backdoor". ie break through with rooks and turn 90 degrees
2/A "Minority Attack", where usually two pawns advance against thee with the idea of 1/ or creating a weak pawn which can be attacked. (EG minority attack in QGD)
3/A "Majority Attack", where three pawns take on two by advancing with the idea of creating pressure by threatening to promote one to queen. This again relates indirectly to 1/as well.
I do understand king side attacking but what is a queenside attack