What is the NEM ?
POI is the consensus algorithm used in NEM to determine who will calculate the next block.
The importance of an account depends on how much money it holds. And how many transactions are made from that account. The POI differs from other initiatives using a cost-sharing model that does not take into account the overall support of the network. In some Proof-of-Stake systems a person only needs large amounts of coins to form a block.
However, in NEM, the amount of the transaction as well as the support of the network become an immortal factor. This was designed to encourage EMS users to not just keep NMS. But rather to actively trade in the NEM ecosystem.