Plain English Meaning
When you're flying alone, you're the only one making decisions, so you have to deliberately use everything available to help you fly safely -- the instruments, the autopilot, charts, ATC, your training, and your own attention. Single-pilot resource management is the skill of doing that well.
Full Definition
The art and science of managing all available resources -- both inside and outside the cockpit -- to ensure the safe and successful outcome of a flight when operating as the sole pilot. It covers the gathering of information, analysis of that information, and decision-making by the pilot, including risk management, task management, automation management, controlled flight into terrain awareness, and .