If you are not continuing from the previous articles, better start there or all these concepts will be uneasy to grasp.
Here is the Link -> Blockchain Fundamentals
We have already learned about the blockchain about how it's chained and about hashes, Now let's learn some more concepts around blockchain.
Immutable Ledger
Imagine you are buying a piece of land with some sum of money, You pay the money to the landowner in exchange for the land, Now how do you make sure that the land is yours? What proof do you have that you bought it? Well, For that we have a registration, We register our properties with the government, The government then maintains a ledger that records these transactions and maintains the record of the ownership of the properties. Now in some cases these ledgers are digital and in some, they are physical books, they are stored in some government building, Since they are stored in a single location, If someone were to go and change the record that says you are the owner of the land and change the ownership, you would no longer own that land since the record has been changed, Therefore, Tampering with the records is quite easy.
Now, let's see how blockchain can help
If all the ownership records were blocks on a blockchain, The Tamperer would not be able to tamper with the blocks easily because as soon as the data of the block will change its hash will change along with it, to maintain linking of the blocks the tamperer then would have to change the (Prev. Hash) property of the next block and then the block after and so on and on and on...Which is quite difficult
But it is not impossible to change, It will take time but it can be done and Hence, We need a system in place that prevents this from happening and so we use a P2P network
Distributed Peer to Peer (P2P) Network
It refers to the network of the group of computers that have a copy of the blockchain on them, All the computers make sure that all the blockchains are always the same and official changes to the blockchain are always in sync i.e Addition of new blocks, In the network, if one blockchain on a computer gets tampered with all the other computers in the network will in some time due to syncing feature, copy the original copy of the blockchain to that computer and will get back that computer the original unchanged blockchain, The person who tried to change the blockchain will not even have enough time to link other blocks together before the original blockchain gets copied.
In order to officially tamper with the blockchain, the tamperer will not only have to change the data and link the blocks on the blockchain in a very little amount of time, but the tamperer will also have to change all the blockchains on at least 51% of computers on the network, that too at the same time, Which is quite impossible.
This is how P2P Network adds security to the blockchain.