Before you play a round you guess how many of the 13 ‘books’ you will win. Books come from the single cycle where everyone throws out one card. The winner collects the 4 cards like a little book and count it as a point.
If you win more books than you guessed, you collect one sandbag for each book overguessed. Every x (usually 10?) sandbags and you get a permanent penalty against your total score.
Reneging is almost the opposite. If you win less than you guessed, you get a penalty instead of adding anything to your score that round.
They’re suggesting that if they guess first, they will purposely make their guess less books than they know they have (and collect sandbags) to hopefully trick their opponent to guess more. It can be easy for the last person who guesses to just subtract what’s been claimed from 13 for a good ballpark of where they should guess, so if there’s a lot left it’s easy to bite off more than you can chew.