Let's say there are 4 voters: A, B, C, D, voting on policies 1-10. Everyone gets a certain number of votes (say one per policy, so 10 total).
Voter A only really cares about policy 5 & 7. So they want to buy votes for these 2. They can give up their votes in, say, policies 1-4 to get 2 more votes for policy 5, and can also give up votes 6, 8-10 to get 2 more votes for policy 7. So they put 8 votes into the "common pool" in order to ensure that, in addition to their vote for 5 & 7, each policy also gets 2 extra votes?
FWIW, the paper describes it as a vote currency that can only be used for the vote, as opposed to cash you could spend on anything. The voice credits have to be distributed to voters.
'individuals pay for as many votes as they wish using a number of "voice credits" quadratic in the votes they buy.'
I now recall that sqrt(1) = 1. I did not recall that 2 minutes ago.