Dhyana can be seen as the penultimate meditational stance, a preliminary for Samadhi, a format for removing unnecessary thoughts, a perfected concentrated state, a devotional divine form of meditation, contemplation of the universal spirit, or remaining concentrated on internal matters whilst shutting off the process of creating wild thoughts, unreal desires, and currently unnecessary matters. It all depends on which brand of Yoga you practice, and your personal belief. In Raja Yoga for an example, Dhyana is a pure thought state where one is absorbed into the target of meditation.
I sometimes find a similar state comes about when I am listening to music. My heart-mind (desires and wants that are for fun rather than necessity) and my think-mind (thoughts, memories, plans) disappear, they remain silent and stay so, whilst my entire focus remains on the music I am hearing. Maybe it is a tune that I have heard many times before, so I am concentrated on a specific track, such as picking out the bass or drums from amongst the whole tune. Sometimes, I will be climbing into the melody or the lyric, totally concentrated on this with no part of my mind functioning on any other matter. From this experience, it is somewhat easier for me to simply transfer what I have already experienced with music into my Yoga practice. I simply alternate the object of meditation from 'music-sounds' or 'vocals' and target my concentration onto a more appropriate matter for Yoga, such as my 'know-mind', otherwise labeled soul, inner wisdom, Buddha nature, clear light, inner divinity, etc.
Pratyahara can be described as the withdrawal or control of the senses in order to still the mind. It implies disconnecting the senses from external stimulations and sensory distractions, in order to retain an inner peace, clarity and progression. Whereas normally we would follow our wishes and desires, by practice of Pratyahara we learn to ignore the level of the mind that constantly wants this or wants that, so as to concentrate our efforts on more important mental activity. It relates somewhat to the Buddhist 'non-attachment' belief, which I mention as that is where I first encountered this matter and where my own practice began. I have personally learned to disconnect from my heart-mind level, in order that my know-mind level will tell me what I truly do need, rather than allowing my heart-mind to talk me into wanting, and then demanding, whatever I encounter that is pleasant. In both Yoga and Buddhism, the controlling of the 'heart-mind' to allow only our natural needs to become wants, and true happiness and contentment to become our pleasure, leads to a more stable, centered, and balanced life. It prevents the heart-mind wanting every new delight it spots, leading us into temptations, desires, and the possibility of envy, greed, impatience, and jealousy.