"in perpetuity" means "forever"
Like my esteemed colleagues have already stated, property law generally does NOT want to allow dead people to exert control over land for years and years and years. (also called "dead hand control"). Societal and economic changes can make such restrictions REALLY burdensome on the current property owners.
Here's an example...
Let's say the year is 1901 and Mr. X owns some land and a house on that land. He writes his will, leaving the land to his son, X Jr. "and his heirs" (a way of leaving property to X Jr. and his heir, then to X Jr.'s heir's heir, so on down the line). But then he continues, writing that the land must be used for residential purposes only, and only single family residences. If this condition is not met, then the land is to go to Y and his heirs.
Fast forward to 2001...
In the century that passed, the land surrounding the X estate has gone from large rural tracts of land to smaller farms to even denser habitation. The area has experienced some major industrialization and growth. Now in 2001, X's land is in a huge metropolitan city. The will says that the land can only be used for single family homes, but all the surrounding land has skyscrapers on it.
Clearly the land would be much more valuable if it could be developed to match the area. Assuming the X family has kept the land all this time, they are really tied down by the will. Also, it will be nearly impossible to sell the land with the current restrictions. The city also suffers a detriment in that it cannot benefit from the best use of this piece of land.
There are more problems - let's say that the X's just give up on owning the land and decide to violate the restriction on the use of the land. So now, according to the will, the land is supposed to go to "Y and his heirs." Chances are, 100 years later, Y is long dead and getting chewed up by worms. Y's children are dead. Any heirs that are alive have likely moved, married, have different names, etc. It could be enormously expensive and difficult to track them down, if possible at all.
The "Rule Against Perpetuities" strives to avoid these problems by limiting the length of time that "dead hand control" can operate. It's EXTREMELY COMPLICATED and generally sucks to analyze, but you use a basic measurement of time to see if the restriction can operate in a way that a third party, like Y, can get the land. This basic measurement is "a life in being plus 21 years."
So what the HECK is a life in being? It's someone who is alive at the time of the grant/gift/transfer (in this case, the writing of the will) - a human measuring stick. Basically, you take one of the parties involved and when they die, wait 21 more years. If it's possible that Y might not get the land before that time is up, then Y can NEVER EVER EVER get the land. His "interest" in the land is TOAST.
Example: X Jr. dies in 1925 (of course in a freak accident involving a runaway horse, a barrel of gunpowder, a grand piano, and a sudden tornado). So you add 21 years - now we're at 1936. And guess what, the X kin are still happily living on the land, in a lovely single family home. Y is SH** OUT OF LUCK. No land for Y!
Here's the fun part - you don't have to ACTUALLY wait and see if everyone behaves themselves for that "life in being's" entire life and then for 21 more years. You get to make up your own story. You can pick ANYONE alive at the time of the grant and kill them off in all kinds of fun ways, add 21 years, and then see if it's HYPOTHETICALLY POSSIBLE that the condition might not happen.
Basically, the Rule Against Perpetuities functions to prevent any conditions (that if violated, send the land to certain third parties)that say the land must ALWAYS be used some way or NEVER be used some way. i.e. "so long as liquor is never consumed on the land", "to be occupied only by members of the Such-and-Such Church", "to be used for residential purposes only", etc.
It's just perfect for those late-at-night-I've-had-way- too-much-coffee/alcohol/other semi-illicit stimulants-philosophical discussions. See my "unborn widow" post above.
Sounds complicated, right? I gave you the Cliff Notes version. The real thing and all its little details are so insane, that if you mess it up when you're an attorney, you are NOT liable for malpractice. Normally if you screw up a legal analysis, you're totally liable. But the courts recognize that this stuff is just too crazy.
Small consolation for those of us that will be tested on it! ARRRGGHHH!!!
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