Please explain WalletExplorer.com layout of data I am having a hard time understanding exactly what information is being presented here:
https://www.walletexplorer.com/wallet/Poloniex.com
I believe that this is the aggregating wallet held by the exchange Poloniex. On the left hand side (received) are incoming bitcoins to the aggregating wallet likely from individuals who are selling their bitcoins to the exchange in order to cash out. On the right hand side (sent) are outgoing bitcoins leaving the aggregating wallet likely due to individuals who are buying bitcoins from the exchange. 
If this is correct, why is the balance decreasing when coins are received? The balance appears to be denominated in bitcoin, but exactly what does this balance mean?
You may try c-hound.ai for a more intuitive interpretation of wallet data. It goes a bit further compared to most common wallet explorers, by highlighting inter-address relationships, token balance, and more. I hope my suggestion will come handy for you.