Concurrent (Merriam-Webster): operating or occurring at the same time

Concurrent programming is simply programming that involves more than one event happening at a time, in the sense that we think of events in a program happening. In a non-concurrent program, if we wanted to perform two calculations, we would perform one, and then the other. In a concurrent approach, we might spawn two threads, and assign each of them a calculation to perform. A big idea in concurrent programming is having multiple processes running at the same time. You can think of it like your computer running Firefox and Spotify at the same time.1

On a hardware level, one way to implement concurrency to have multiple CPU cores (processors, the chips that do the math). Thus, we can add two numbers on one core while dividing two numbers on another core.

1 Your computer might actually just be switching between the applications really fast if you only have one CPU core, giving the illusion of multiple processes happening at the same time. Even if you have many cores, it's possible that the the applications could be running on the same core. It's all up to the task scheduler.