The confirmations of the Bitcoin network are a necessary process to guarantee the finality of transactions. Once a transaction is sent, it needs to be included in a block to be considered as legitimate and have the guarantee that there will not be any double spending attacks.
The number of confirmations of a transaction represents the number of blocks that have been generated after the block where that transaction has first been included. The more confirmations received by a transaction and the safer it can be considered.
Merchants that wish to accept bitcoin payments can set the number of confirmations necessary to consider a received payment as valid and final, with a trade-off between velocity or security.