Continuing on from the who gets what of interpreting the last will and testament of you deceased British or Irish relative.
A life estate is an interest in land for the duration of the owner's life. This is the simplest of all estates, as when you die, you stop owning it.
So what happens to it after you die? Not that you care, you're dead, but those greedy brats you left behind will probably want a piece of it, right?
Assume X owns an estate in fee simple. He's a generous guy, despite the mysterious name, so he grants a life estate to Y - "Whiteacre to Y, for life". When Y dies, the land reverts back to X, why?
Because X has a fee simple tenure and owns the land forever, for eternity, Y merely has it for life. So X is the reverter.
To put some spin on the process, a testator will often say "All my land to Z for life, thereafter to A in fee tail". This means that Z inherits the land, but when he dies, it reverts to A, and then passes on to A's children (they call them "Issue" in the courts in order to further the impression that butter wouldn't melt in their mouths).