They are difficult to understand, at least initially because they follow a different system and use different terminology that doesn't translate precisely into English.
But, here we are going to attempt to break them down, bit by bit and relate them to western concepts that you may or may not be more familiar with.
Is your intention. This one is fairly straightforward because it translates nicely. It is a thing intended, it's an aim or plan. You can use your intention to drive your behavior and actions, just by holding a desire in your mind. It doesn't mean it will always lead to that result, for example you may intend to bake a really delicious apple pie, but you might botch it and burn the crust. But you start with an intention. A common saying in kung fu is, "Yi leads qi, qi leads li." Or in English, "Intention leads energy, energy leads strength". Yi is on the third (mental) level of the 5 Levels of Human Potential
Is your body's energy. Three things affect this - intent, breath, and movement.
Qi flows more easily through relaxed muscles, it is led by intent (yi), and it circulates more forcefully when propelled by reverse breathing or "compression breath".
Although there isn't a similar model in western medicine for qigong and acupuncture for how qi flows in the body. There are remarkably similar systems that are being used that seem to be working along the same planes.
I will share a personal anecdote about qi and another energy system used in the medical field.. When I was in Texas, I went to a chiropractor that used a technique that I had never heard of before. It was called Network Spinal Analysis (NSA). It was basically an "energy form of chiropractic care" as they have a continuum in their methodology in chiropractic schooling.
The method is described as:
"Gentle precise touch to the spine cues the brain to create new wellness promoting strategies. Two unique healing waves develop that are associated with spontaneous release of spinal and life tensions, and the use of existing tension as fuel for spinal re-organization and enhanced wellness." - via Wise World Seminar
Tony Robbins also explains his experience with NSA:
(video contains profanity)
Is muscular strength. This concept isn't so much debated, but becomes a topic of debate when qi gets involved. In kung fu, they often refer to qi as a driver of strength, "using your qi" so some people may simply avoid that concept and look at plain body mechanics and the endocrine system, like adrenaline. However, in qigong, they explain that qi comes from your endocrine system. So, the concepts are inextricably connected.
Is power, it is manifested qi/li I've noticed that the way they use this term is somewhat different than "li" or strength. For example, if you were off balance, they would say your qi couldn't penetrate into your muscles to create jin, or power. In that state of being off balance, you would feel weak rather than powerful. So, it describing a feeling as well as their concept.
So, if all these concepts seem to get confusing for you, it may be easier to simply approach it as a feeling. Feelings, emotions, and energy are on the second level of the 5 Levels of Human Potential. In this area, it is a little more abstract than body mechanics, but describing it in a way that makes sense to you can help you see the benefit of the exercises and practice.
If you are skeptical about Qigong, I wrote an article explaining The Truth About Qigong and I go into depth explaining some of the concept in a more western speak.