When you run out of hardware memory, an advanced operating system will free up memory space by moving the contents of some of your memory to disk. When that information is needed again, it is read from disk back into memory. When your computer is thrashing, memory is being swapped back and forth at such a rate that your hard drive is constantly spinning and not much else is happening with your computer.
To move wildly or violently, without accomplishing anything useful. Paging or swapping systems that are overloaded waste most of their time moving data into and out of core (rather than performing useful computation) and are therefore said to "thrash". Thrashing can also occur in a cache due to cache conflict or in a multiprocessor. Someone who keeps changing his mind (especially about what to work on next) is said to be thrashing. A person frantically trying to execute too many tasks at once (and not spending enough time on any single task) may also be described as thrashing.
In computer science, thrash is the term used to describe a degenerate situation on a computer where increasing resources are used to do a decreasing amount of work. Usually it refers to two or more processes accessing a shared resource repeatedly such that serious system performance degradation occurs because the system is spending a disproportionate amount of time just accessing the shared resource. Resource access time may generally be considered as wasted, since it does not contribute to the advancement of any process.