Processes are said to be coupled when they are tightly wound together, and dependent on one another.
A loose analogy might be replacing a traffic light by a traffic circle. You keep the traffic moving, reducing the overall wait time for any car entering the intersection.
Decoupling web applications might involve replacing a makeshift queue your application currently implements in a table, with a message queuing service such as RabbitMQ or Amazon’s SQS.
Ultimately decoupling promotes scalability, as you can scale the pieces of your infrastructure that your capacity planning identifies to be bottlenecks. What’s more you can make those pieces redundant, increasing high availability at the same time.