What makes Tai Chi an internal art? Most books on the subject will tell you it's something to do with energy and the mind. Whilst there may be some truth to this it doesn't help a beginner very much.
Tai Chi is internal in a very physical sense. When you align your skeleton in a specific way and relax your muscles your internal connective tissue or fascia will activate. In most people, this internal strengthening is initially hard work as you're exercising deeper parts of your anatomy that are probably quite weak.
I've heard fascia referred to as postural muscles which is a helpful definition because the fascia is what allows you to stand. It is the connective tissue that joins your muscles to your bones and runs throughout your body. It is very important to keep your fascia healthy but most forms of exercise don't. You can only strengthen it by relaxing. Lifting weights and doing press ups may serve some purpose but these types of external exercises will hinder any kind of internal work.
The reason relaxation is emphasised above all else in Tai Chi is for this reason. Most people are quite tense and those that go to the gym regularly are likely to be even tenser. It seems that when you over exercise your big muscle groups you are doing so at the expense of internal strength.
Tai Chi is a martial art that seeks to train in whole body strength. Eventually, you also want to strengthen your muscles but to place too much emphasis on that kind of training, in the beginning, would severely impede progress.
Tai Chi develops a deep internal body connection. When I move I can feel the connection between my hand and my foot. This starts as an idea, becomes a faint sensation and eventually becomes a deep physical reality. When you start to develop this feeling Tai Chi practice becomes very satisfying indeed.