This was a question I have been asked a few times, especially when a learner wants to be able to paint something specific like hair for example. Learners can get very hung up about techniques and how to learn them. Techniques need to be thought of as ways to apply or remove paint to create a visual effect, that is all they really are. Some techniques are extremely simple although at first glance you may not be able to tell how the painter achieved the effect. Some techniques are more complex and require more of a knowledge of materials.
Life drawing is drawing (or sometimes painting) of the human figure from a live model. Sometimes the model is clothed or semi-clothed but the idea is that you can observe the human form in its ‘life’ or natural form.
Drawing clothed models is very useful of course because you can get to observe the folds and creases of the clothing. Life drawing has the advantage of seeing the anatomy as it is and sometimes to understand how clothing behaves in the way it falls and creases we have to understand what is underneath, the anatomy.