Whenever you send someone cryptocurrency, the transaction goes through different computers running the protocol around the world that make sure the transaction is valid. Once the transaction is verified it then “waits” inside the pool.
Once inside the pool, it waits to be picked up by a node or a miner and entered into a block of transaction on the Blockchain. Until it is picked up it’s considered an “unconfirmed transaction” or a “pending transaction”. A new block of transactions in added to the Blockchain every 10 minutes on average. However, today there are so many cryptocurrencies, and a block can only hold a finite amount of transactions, not all transactions are selected instantly. Consequently, you will need to wait until a miner or a node decides to pick your transaction out of all of those sitting in the pool.
Once a transaction is included in the block it receives its first confirmation and no longer has a pending status. After another block of transactions is added it will get another confirmation and so on.