Answer Description
Processors run calculations using threads and each processor core can only handle one thread at a time. Additional cores improve processor performance by enabling multiple threads to be run simultaneously. When an application uses multiple threads on a single core processor the threads do not execute simultaneously limiting performance.
Wikipedia
A multi-core processor is a computer processor integrated circuit with two or more separate processing units, called cores, each of which reads and executes program instructions, as if the computer had several processors The instructions are ordinary CPU instructions (such as add, move data, and branch) but the single processor can run instructions on separate cores at the same time, increasing overall speed for programs that support multithreading or other parallel computing techniques Manufacturers typically integrate the cores onto a single integrated circuit die (known as a chip multiprocessor or CMP) or onto multiple dies in a single chip package
Multi-core_processor - Wikipedia, the free encyclopedia