Technically, when a person makes a bitcoin transfer, this transaction is released to the Bitcoin network to be processed and verified by all the computers that integrate the network. When this transaction is included in a block from the blockchain it is said that this transaction has a confirmation, when it is included in the following block it is said that it has 2 confirmations and so on. 

In other words:

A confirmation means that a transaction has been processed by the network and it is not likely to be reversed. The transaction are confirmed when they are included in a block and for each following block. Each confirmation exponentially reduces the risk of the transaction being reverted. 

In Bitso 4 confirmations are necessary in the network for your Bitcoins to be available, this normally occurs in approximately 20 to 30 minutes.