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In this case, dust refers to the minimum payout value on the blockchain.
Any Post/Comment not getting above $0.02 after 7 days will be worthless. So if your comment is worth $0.019, you will get nothing...
That is the dust @dustsweeper is saving.

i don't understand you line 'if your comment is worth $0.019 you will get nothing '

I upvoted your comment now, so it is worth around $0.01. If that will remain like that until the 7th day, nothing will be paid out.

There is a built-in threshold in the blockchain that prevents pay-outs of less than $0.02. That is the whole idea behind creating Dustsweeper, to help protect these small profits.