In order to confirm the identity of the person entering an account, a verification code is given to a different device. It's used for two-factor authentication.
Falling Knife
The term falling knife (also known as "Catching a Falling Knife" or "Catching the Bottoms") refers to the action of buying an asset which is rapidly declining in price.
In a few words, a candidate block is a block that a mining node (miner) is trying to mine in order to receive the block reward. So a candidate block may be described as a temporary block that will be either validated or discarded by the network. Miners compete with each other to validate the next block and add it to the blockchain, but first, they have to create a candidate block to participate in the mining competition.