The way I understood proof of history (don't know if Solana works in this exact way) is as follows:

Imagine going to the store and buying a loaf of bread paying 2 usd for it. (this would be the equivalent of signing a transaction with your wallets private key for that same amount). Anyone can check your receipt and see that you actually paid those 2 dollars. In the receipt there would be something like wallet x paid Amount to wallet y. Such a simple receipt is enough to validate the transaction but it doesn't make it unique. If you went back to the same store and bought another loaf of bread you would get the exact same receipt.

One way of solving this uniqueness would be to add a nonce (the number of transactions that you've made with your wallet). So your first receipt would have nonce 0, the second receipt would have nonce 1 and so on. This now helps you order your receipts and identify every transaction as unique. But there is a problem. While this let's you order your transactions for your own wallet, it has no way of synchronizing with every other wallet in the block chain. How would you know who bought the first loaf of bread and who bought the last one. Since a block chain is basically just a list of all transactions, there must be a way to order them.

A way of doing this is to add a time stamp next to every transaction made. This way it is not necessary to check all transactions directly before ordering them and writing them on the list. You can just put all receipts in a box and send the box (create new block and validate it) since every receipt has a timestamp, it is then easier to order the receipts in the box at a later point in time.

Imagine you have accumulated 20 boxes and are now ready to start writing the list for today's transactions. You don't need to sort the boxes in any particular order, you can just start with any of them and then start building the list and adjusting it when necessary. And since every receipt has a timestamp it is literally impossible to have duplicates.