A process is a sequential order to executing tasks. A process can be super long with loads of tasks or short with just a few. Almost anything can be thought of as a process. For example, going into work can be process, or grabbing a cup of coffee, or your morning routine.
Processes don't stay fixed. You can have an initial process but change that process as you make improvements on efficiency. In fact, the best processes are determined after making a lot of iterations, and even then the best processes can be made better.
Process improvement is the mindset to make processes better. It describes a constant focus to try to squeeze efficiency and performance out of a process. It can be an obsession. It requires constant iteration.
Focusing on the process and process improvement is the key to high performance. The world's best athletes go through the same routine when setting up shots or going into tournaments. They focus on getting the process right and not the outcome. They trust the process.
Focusing on the process and not the outcome doesn't mean that you don't care about the outcome. It just means that you know the process will get you to the outcome, and the only thing you can control is how you execute the process.