Memory barriers

Memory barriers ensure ordering of load/store commands.

Usually CPUs and compilers reorder these instructions for more efficient execution. Lock-free algorithms often require that perceived execution order be strict, that's when memory barriers or memory fences come in handy.

More detailed description in following articles:

Memory Ordering in Modern Microprocesors, Part 1 and Part 2

Ridiculous Fish's controversial article about using and abusing memory barriers. (and bashing of this article just for a different POV)