When a miner solves a block, they receive the block reward plus fees. These are newly minted bitcoins that have never been spent (transacted) on the network. Bitcoin's finite supply of 21 million coins also presents an interesting aspect to First Spend; when there are no block rewards left, then there will be no new coins for First Spend to record.
Before bitcoins are spent, they show up on ByteTree as unspent inventory, which we have shortened to inventory; when they are transacted for the first time, they are listed as First Spend. Each subsequent time they are then spent in the future, they are measured in general spend or transaction value.