Sweat is how the body keeps its temperature in check. Then, glands across the skin release moisture, it evaporates, and you cool down. In hyperhidrosis that system fires when it has no reason to. People with the condition sweat heavily while they are cool, still and calm. This happens not only when it is hot or they are nervous. Still, no amount of willpower switches it off.
Doctors sort it into two kinds. Primary focal hyperhidrosis is the common one. It centres on specific areas, usually the underarms, palms, feet or scalp. It often runs in families. In addition, it tends to begin in childhood or the teenage years. Meanwhile, secondary hyperhidrosis is sweating brought on by something else, a medication or an underlying health condition. As a result, it usually affects the whole body rather than a few spots. Telling the two apart matters, because the second kind needs its cause found and addressed first.
Roughly one in twenty people live with some form of it, and many go years assuming nothing can be done. In practice, people often apologise for it, as though it were a habit rather than biology. Yet, it is a genuine medical problem, not a matter of hygiene or nerves. In almost every case it can be reduced dramatically. You can read the plain-English overview on the NHS hyperhidrosis page.