CPU is the acronym for Central Processing Unit.
A Central Processing Unit is the main processor in any computer, the electric circuitry that executes instructions that make up the computer program. CPU’s performs basic arithmetic, logic, controlling, and input/output operations.
In the context of cryptocurrency and blockchain, CPU’s were the original devices & hardware used to participate in the Bitcoin Network. Circa 2009, 2010 & 2011, it was possible to extract 50 BTC per block mined and all that was required was a basic $300 laptop.