State of confusion: What is LAMP and when can I say I know it?

I've been learning PHP a bit over a year now and am getting fairly confident with it. Am at the slightly pre apply for jobs confidence level but nearly there.

One thing that confuses me is many jobs state you need to know the LAMP stack..

I'm able to get things done with MySQL and PHP and I've assumed my webhost works on a Linux Apache basis, but I couldn't really say I know anything about Linux nor Apache.

Guidance thrown my way would be appreciated, what does knowing LAMP mean and how would I get from here to there?