I have foudn this site commenting on the Yoga Sutras to be helpful at times.
Meditation on the subtle: It is very important to reflect on the principle of meditation on the subtle elements. Meditation at this stage means that you are dealing with the very building blocks of all of the objects on which you might meditate in their gross form. You are focusing not only with objects normally seen to be external (the things of the world stored as memories in the mind), but also the very instruments (such as senses and mind) by which those objects are experienced. In this way it becomes increasingly possible to attain non-attachment to the whole realm of gross matter, along with their subtle counterparts and the mind itself.
It seems to me very simple thought, or pure awareness of just being there, asmita, I-am.
Gross Objects: The notion of meditation on a Gross Object is pretty straightforward, including any of the familiar Objects, such as visualized images, sensation, breath, mantra, and attitudes, etc..
Subtler components: All of the Objects are constructed of five Gross elements called bhutas, and these are earth, water, fire, air, and space. These five also have five Subtle elements, called tattvas, which are the Subtler aspects of earth, water, fire, air, and space. Subtler than these are the mental processes and three components out of which these arise, which are called gunas (sattva, rajas, and tamas).
Although one may be practicing meditation on Gross Objects, which is extremely useful, it is also important to recognize that these Subtler explorations of the component nature of the Objects is a further stage of the meditation process. The particulars of how to do those Subtler meditations come with practice and training.