This is a great question and unfortunately the answer is a little bit broad because a tonic’s action is not so specific. Tonic herbs strengthen and nourish and invigorate certain systems or tissues in the body. They tend to be herbs that are more food like and need to be taken long term to have the effect that is desired.
I hope this helps a bit, but don’t worry if it isn’t completely clear just yet, you are just starting your journey into herbal medicine, which can be quite different from the health and medicine from our current every day care, which tend to be reductionist.
As you go through the course and start working with the herbs, concepts will likely start to fall into place.