CPU Cores are physical or logical units within the CPU responsible for executing instructions. 

Physical core: It's the hardware, its whats actually on the CPU

Logical core: is the code basically, and its also known as a Thread. Logical cores improve CPU performance by allowing multiple instruction streams to be executed in a single physical core

Above is a 4 core CPU(Central Processing Unit) and they are responsible for executing instructions. In the image above, let's go on with the fact that there are 4 physical cores, but lets say we have 8 Threads, then that means that each physical core has the ability to execute two instruction streams simultaneously