What is a thread, as in multi-threaded?

Multi-threaded means performing multiple operations at once. Depending on the capabilities of your computer's processor, this may be "simulated threads".

For example: a single-core processor that has one real thread can only really do one thing at a time, but simulates multiple threads by starting/stopping each simulated thread in short increments, giving the illusion of multiple threads. A dual-core (or higher) or single-core (with two or greater real threads) processor can perform multiple tasks at the same time.

Any time a program allows you to set a number of threads for operations, keep simulated threads and your CPU's capabilities in mind. Most of today's CPUs are fast enough that you'll almost never notice these "simulated threads". Performance bottlenecks are usually noticed with a hard disk or internet connection with newer CPUs.