Unconfirmed transaction means that the transaction has not been included in a block and thus has not been completed. Most recipients require at least one confirmation to complete the transaction, including Wirex.

Common reasons for unconfirmed transactions are:

You have just made the transfer. The Bitcoin network needs at least 10 minutes, on average, to include the transaction in a block;
The blockchain fee is too low or absent. The lower the blockchain fee, the lower you transaction's priority in the Bitcoin network. Therefore, the longer the confirmation will take;
The Bitcoin network is overloaded. Due to its limited capacity, the network simply cannot process transactions fast enough, especially during high transaction periods.
If your transaction is unconfirmed, usually you just need to wait. Most transactions get completed within a day or two (or more if the blockchain is overloaded).