Lightweight and heavyweight processes refer to the mechanics of a multi-processing system.
In a lightweight process, threads are used to divvy up the workload. Here you would see one process executing in the OS (for this application or service.) This process would possess 1 or more threads. Each of the threads in this process shares the same address space. Because threads share their address space, communication between the threads is simple and efficient. Each thread could be compared to a process in a heavyweight scenario.
In a heavyweight process, new processes are created to perform the work in parallel. Here (for the same application or service), you would see multiple processes running. Each heavyweight process contains its own address space. Communication between these processes would involve additional communications mechanisms such as sockets or pipes.
The benefits of a lightweight process come from the conservation of resources. Since threads use the same code section, data section, and OS resources, fewer overall resources are used. The drawback is now you have to ensure your system is thread-safe. You have to make sure the threads don’t step on each other. Fortunately, Java provides the necessary tools to allow you to do this.