Every transaction must be added to a blockchain — an official public ledger of crypto transactions, in order to be considered successfully completed or valid. Every bitcoin, bitcoin cash, ether and USD PAX transaction in your Blockchain Wallet flows into what’s called the mempool (short for memory pool) before it can be confirmed by miners. To learn how Stellar is confirmed, read more here: https://support.blockchain.com/hc/en-us/articles/360019105391-Stellar-consensus