What rucking actually is
Walking or hiking while carrying a weighted backpack (a ruck), originally a military training practice, now adopted by a broader fitness audience seeking a combined strength-and-cardio training stimulus.
The genuine physiological rationale
Adding load to walking increases cardiovascular demand and engages muscles (particularly the core and legs) considerably more than unloaded walking, providing a combined strength-and-cardio benefit from a relatively simple, accessible activity.
Why it's gained mainstream popularity beyond military and tactical fitness communities
Rucking offers a genuinely low-impact, equipment-minimal way to add meaningful training intensity to simple walking — appealing to people wanting more challenge without the joint stress of running or the equipment needs of a gym.
Getting started safely
Starting with a modest weight (often recommended around 10% of body weight for beginners) and gradually increasing both weight and distance, using a properly fitted backpack, helps avoid the back and joint strain that can come from starting too aggressively.