MemoryThrashing is a degraded case of swapping. The term "Thrashing" means that every time a process wants to use memory it has to swap some data out and some more in, meaning that your machine runs very very slowly. Technically it means that your "Working Set" is larger than your "Physical" memory.

You can usually tell this by having processes listed in ps(1) and top(1) as being in the "D" state (busy waiting on I/O), and by looking at vmstat(1)? and seeing that the "si" (swapin) and "so" (swapout) columns are high. Also the system % CPU usage as shown by top(1)'s summaries will be high.

Solution to this problem? Buy more memory, or use less of it :)