The initial blockchain download is the process of downloading the complete blockchain data of a specific cryptocurrency or blockchain. This data download is necessary in order to interact or transact with that particular blockchain. The initial blockchain download can take a while and is quite resource-intensive depending on the blockchain or cryptocurrency being downloaded.
When you initialize a wallet associated with a specific blockchain, such as Bitcoin, the wallet software will go through the process of downloading the entire blockchain of that particular cryptocurrency. This includes downloading all past transactions and confirming whether information is in line with the current state of the blockchain.
In some blockchains, like Bitcoin, this includes downloading and verifying every single past transaction on the blockchain. This can take some time depending on the speed of your internet connection. When you download an initial blockchain, you create a local copy of that particular blockchain’s database.
This database contains information about all blocks and transactions, which are used by wallet programs to calculate and validate balances as well as to simplify other processes such as confirming and validating incoming transactions. After the initial blockchain download is finished, a certain amount of data will be stored on your hard drive.