Unconfirmed Transations in the mempool
Unconfirmed transitions live in the mempool. When you transfer funds to a different wallet address, the details are stored in the mempool before they are permanently committed to the blockchain.
Each running full node on the Bitcoin network contains this mempool.
Each transaction inside the mempool is called a block. The mining machines try to solve a math problem, the first to solve the problem gets to add the block to the block-chain. The block-chain is the digital ledger which permanently stores all transactions.
The block time is 10 minutes, but it can take considerably longer to add blocks. Particularly if miners are not picking out the transactions from the mempool. This generally happens when the fee is set too low.
Bitcoin miners are always looking to find new bitcoin blocks. When they do, they get a reward of 12.5 BTC. But apart from this reward, miners also get the transaction fees or sometimes called a “Bitcoin mining fee”.
You can check and see how busy the Bitcoin network is before doing a transfer here: https://blockchain.info/unconfirmed-transactions
But most importantly you need to set a higher mining or transfer fee to make sure it goes through quickly.