If you're not sure what that stands for, it means "Live long and prosper". It's a saying the late Leonard Nimoy said as his character Spock on the tv show Star Trek. It was his catch phrase when he was wishing someone farewell.
It became more than just a catch phrase for a imaginary character on a cancelled tv show, it became a house hold phrase. Mr. Nimoy adopted it in regular use, much like he uses in his twitter stream. It's been plastered everywhere, on mugs, t-shirts, posters, and just about any other material possession you can think of.
The phrase itself is a simple one. Live long and prosper. It means to live a long life and may you be successful in anything that you do. Leonard Nimoy was the personification of this phrase. He lived a full life and accomplished a lot. As of now, he passed on.
His character Spock would say that he simply stopped being. There's no scientific proof in an afterlife and his character is grounded in the evident and provable. Religiously, his spirit moved on to meet his maker. Philosophically, you exists as long as you are remembered.
I think Neil Degrasse Tyson put it best. The simple fact that you existed in this universe, the memory of time will remember you forever. What that means is subjectively up to the reader, but I take it as the immortality we all seek. I exist here and now, I made changes in this universe and that is enough for me to know the universe will remember me forever.
So to Leonard Nimoy, an actor, a director, a poet, a philosopher, a teacher and a father, where ever you are now, you are mourned, missed and respected by many, and by myself as well.
From one of your fans, may your memory live long and prosper.