Block depth is the number of blocks between your transaction and the latest block in the blockchain. Transactions included in the first block after the transaction are said to have a block depth of 1, transactions included in the second block are said to have a block depth of 2, and so on.
For example, let’s say Bob receives five bitcoins from Alice. Bob’s Bitcoins would have a block depth of 1 if included in the following block to be mined. If they were included in the block after that, they would have a block depth of 2, and so on.
The block depth is significant because it determines how quickly a transaction can be considered secure. A transaction with a block depth of 1 is not safe because it can be reversed if someone manages to mine another block before anyone else does.
When you send or receive bitcoins, you want to wait until your transaction has enough confirmations (block depth) before considering it confirmed. It cannot be reversed by an attacker who manages to mine another block.