Concurrency is the ability of a program to make progress on a task that is spread out over different slices of time. It allows us to run multiple “threads” at the same time. As one thread may be sleeping or waiting on I/O, another thread may take priority and start working, thus making optimal use of available CPU time. When you think of concurrency, think of “threads”.

https://blog.redpanthers.co/concurrent-ruby/