Liminality is the state of being between two states. Like transitioning from one year to the next.
Or from childhood to adulthood -- sometimes, as an additional factor, from unwed to married -- which is among the many reasons that young adults are so prevalent in fantasy literature as protagonists.
Or, of course, transitioning from wolf to man and back. Or being simultaneously both alive and dead. Which may help explain some of the popular tropes in urban fantasy.
And there are all sorts of spot liminality that are useful. Putting the characters at a crossroad, or having events happen at midnight.
Difficult to use well. If you have a liminal character or situation, it's very easy to forget to convey the uncanny, the eerie effect of being neither one thing nor another. But valuable when you remember.