It was politically binding. Each side said so before the vote. People voted on the basis it would be acted upon.
It wasn't legally binding, though, it was advisory only, according to the European Referendum Act of 2015: “because of the sovereignty of Parliament, referendums cannot be legally binding in the UK, and are therefore advisory”. May and the rest used the fact that it was advisory as a legal loophole to stop the referendum being voided, which it should have been, because of the proven corruption and lies by Leave in the build up to the referendum. It was most definitely not the "divine mandate" Leavers tend to insist it is.