A CPU core is a CPU’s processor. In the old days, every processor had just one core that could focus on one task at a time. Today, CPUs have been two and 18 cores, each of which can work on a different task.
A core can work on one task, while another core works a different task, so the more cores a CPU has, the more efficient it is. Many processors, especially those in laptops, have two cores, but some mobile processors, such as Intel’s 8th generation processors, have four. You should shoot for at least four cores in your machine if you can afford it.
Most processors can use a process called simultaneous multithreading or, if it’s an Intel processor, hyper-threading (the two terms mean the same thing) to split a core into virtual cores, which are also called threads. For example, AMD CPUs with four cores use simultaneous multithreading to provide eight threads, and most Intel CPUs with two cores use hyper-threading to provide four threads.
Some apps take better advantage of multiple threads than others. Lightly-threaded apps like games don’t benefit from a lot of cores, while most video editing and animation programs can run much faster with extra threads.
Note: Intel also uses the term “Core” to brand some of its CPUs (ex: Intel Core i7-7500U processor). Of course, Intel CPUs (and all CPUs) that do not have the Core branding use cores as well. And the numbers you see in an Intel Core (or otherwise) processor is not a direct correlation to how many cores the CPU has. For example, the Intel Core i7-7500U processor does not have seven cores.