The question asked in the title is loaded. Many people now may be familiar with Qigong. Qigong is a very modern term for a set of health exercises and calisthenics primarily derived from the Chinese internal martial art of Xinyiquan. Preserved within Chinese internal martial arts are many aspects of the wider culture. Self cultivation is extremely important in Confucian philosophy and this deeply influenced the existing schools of Buddhism and Daoism as well.
The word Neigong breaks down into "inner skill" of your body. Often, people believe they are doing this by waving their hands in the air or through different visualization practices. While this is a part of it, most are playing pretend. It is extremely difficult to find someone capable of teaching true internal development skills.
Damo Mitchell is a fairly young man with more decades of practice under his belt than most in their 60s. I highly recommend his book, A Comprehensive Guide to Daoist Neigong. I received two copies of this for my birthday after it was initially released and could not be more pleased. The information Damo presents is concrete, linear, and progresses logically through a process that few understand to this level. He helped clarify almost a decade's worth of my own practice and experience only to find out I needed to get his book on Mingmen Fire Lighting - White Moon on the Mountain Peak as it had more information related to my current practice level. I have also rather enjoyed Damo's Scholar Sage podcast on YouTube.
For our purposes in practice we have will be using neigong to refer to a process of refining movement so that the small postural muscles in the torso are activated and able to be utilized in movements. Some exercises correspond to specific areas of the body while others will focus on movement chaining or relaxation. A competent fighter should have a maximum usage of their body - muscles are only one part - we learn to use the connective tissues, structure of the body, shift the bones, and breath pressurization in addition to muscular strength. This kind of varied training created a body that is very difficult to counter by those unused to dealing with someone like that. Much of the beginning stages are to get one in touch with the Dantian. The video below is from Yang Hai - one of the true gems of Chinese martial arts in North America.