In simplest terms, PoSt means that someone can now guarantee that they are spending a certain amount of space for storage. 

What Is Proof-of-Spacetime (PoSt)?

Proof of Spacetime (PoST) is a type of consensus algorithm used in distributed ledger technology based on the concept of timestamping blocks of data. The concept of PoST is that new blocks of data are only accepted if they are timestamped with the current “time”. In a blockchain-based system, this is typically achieved through a cryptographic proof-of-work system. 
Proof-of-work requires miners to solve a cryptographic puzzle in order to create a new block, and the timestamp is used to ensure that the new block is accepted by the network. This system is designed to ensure that the system is secure, as the miners must prove their work was completed before the block was accepted.

PoSt, usually used in reference to Filecoin, proves that a storage miner is continuing to store a unique piece of data for the network. It is similar to proof-of-capacity in several ways: they both require participants in the network to have a financial incentive to participate honestly in the network and have no benefit gained by harming the network. However, PoSt differs in that it lets network participants show that they have been physically storing data over a certain period of time. This is proved as randomly chosen miners will be selected and have their data read for verifications.

According to the creators of proof-of-spacetime, Tal Moran and Ilan Orlov, this is called the “'Rational Proofs of Space-Time because the true cost of storage is proportional to the product of storage capacity and the time that it is used.”

Proof of Stake vs Proof of Spacetime

Proof-of-stake (PoS) and Proof of Spacetime are both consensus protocols used in blockchain networks. Both are designed to provide a secure, trustless and decentralized way to reach consensus on the state of a blockchain and the transactions it stores. Proof of Stake rewards participants for validating transactions on the blockchain. It is based on the idea that the more coins a user holds, the more likely they are to act in the best interests of the network. This means that the more coins a user holds, the more influence they have over the network. 

On the other hand, Proof of Spacetime is specifically designed to incentivize users to store data on the blockchain. It rewards users for storing data on the blockchain for an extended period of time. This is done by requiring them to lock up a certain amount of coins in exchange for the right to store data on the blockchain. The main advantage of PoST is that it encourages users to store data on the blockchain, which is beneficial for the network as a whole. 

In conclusion, both PoS and PoST are viable consensus protocols that can be used to securely and trustlessly reach consensus on the state of a blockchain and its transactions.