It is French for a blank sheet of paper, meaning a signed blank cheque for which the recipient decided the amount to be given. In other words an unrestricted licence.
It means to be given full discretionary power, freedom or permission to do anything.
It was originally 'charte blanche' in French and came into English in the early 1700s as 'chart blanch'. It was not specifically concerned with money, but applied to any situation in which the holder of the 'empty sheet' could write out his/her own terms. In more recent years, we have taken on the modern French spelling, as presented in your question.