Unconfirmed transactions are not secure
When a transaction is created, it is placed in the unconfirmed transaction pool (Mempool). If a transaction is successfully added to a block, it will receive a confirmation value. The value increases continuously as the blockchain progresses and increases the probability of the transaction (see table).
Each confirmation lasts between a few seconds and 15 minutes, with 4 minutes being the average. For example, if the minimum fee is selected, it may take much longer for the first confirmation to be issued.