As Mark Pack tweeted last night, that's the percentage of people who according to a question asked in a recent YouGov survey would vote Lib Dem if they thought we could form the next government:
'How would you vote if thought LibDems "had a significant chance of winning the election nationally": LD 49% Con 25% Lab 19% YouGov 18-19/4'
We are already above 30% in some polls. I think it is also fair to say that the momentum is with us. Imagine what could happen if more and more people realised that an historic breakthrough is possible.
Now I don't want to get carried away here. Many activists like me are gobsmacked at how quickly we have jumped from c20% to c30% of the vote and are just hoping that the current numbers can be sustained let alone anything else. However I think this polling question demonstrates that it would at least be possible for us to do even better than we are at the moment.
UPDATE 10:23: Having been asked in the comments I have now been given a link to the data for the numbers referenced above. It is here. (Note: opens PDF)