(I'm turning my comment on Jontia's answer into an answer in itself, not because I think Jontia's answer is wrong, but because I think it doesn't fully explain what's going on.)
Theresa May is dangling the offer of a Parliamentary vote on a second referendum (or 'confirmatory referendum' as some liken it) if Parliament votes through her Withdrawal Agreement Bill when it comes before them.
Many members of Parliament wanted a second referendum to be part of the Withdrawal Agreement Bill, but instead it's being offered as a conditional second vote.
Why? Because Theresa May knows it will not pass as a vote in its own right.
Each time such a consideration has been voted on in Parliament in the past few months, it has been defeated - firstly a number of times as an amendment (14th March, amendment (h) defeated 334-85 for example), and then twice during the "meaningful votes" debacle:
27 March 2019, proposition by Margaret Beckett, "Referendum on the Withdrawal Agreement" was defeated 295-268
1 April 2019, proposition by Peter Kyle and Phil Wilson, "hold a Confirmatory Public Vote" was defeated 292-280
As Jontia says, it's smoke and mirrors - she is hoping that the prospect of a vote on a second referendum is enough to gain votes to pass the Withdrawal Agreement Bill whilst knowing she is taking a very small gamble on the second referendum vote.