What was the liquid given to Harry from Snape when he was murdered in Harry Potter 7 for?

In the Deathly Hallows, Voldemort murders Snape in the Skrieking Shack to get the Elder Wand. A weird gasious blue liquid comes from Snape, and he tells Harry to take it. Hermione gives him a flask..and he well...takes it. I didn't see that liquid mentioned again in the book. Was this a mistake, or is there going to be another book? There might be another answer, that's why I'm asking.