Whats the best way to clean sunglasses?

i bought some new sunglasses today, only cheap ones 15 pounds, the sunglasses came with a cleaning cloth, but it says on the back of the packet to clean with a ' non-abrasive' detergent or spray?

what would a non-abrasive detergent or spray be?

i am in the uk by the way.
Update: thanks to all who answered. helpful.
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