A system's concurrency is the degree to which the system is able to perform work simultaneously instead of sequentially. An application written to be concurrent in general, can execute more units of work in a given time than one which is written to be sequential or serial.
When we make a serial application concurrent, we make the application better utilize the existing compute resources in the system—CPU and/or RAM—at a given time. Concurrency, in other words, is the cheapest way of making an application scale inside a machine in terms of the cost of compute resources.
Concurrency can be achieved using different techniques. The common ones include the following: