At the beginning of practice we traditionally chant the following shloka.  
Vande Gurunam Charanaravinde
Sandarashita swatma sukhava bhode
Nishreyase Jangalakaya Mane
Samsara halahala mohashantiye
Abahu Purushakaram
Shankachakra Asi Dharinam
Sahasra Shirasam Swetam
Pranamami Patanjalim Om

These two shlokas are the opening chants that begin our practice, they are often referred as the Ashtanga Invocation.  I get asked (a lot) about the meaning of the Ashtanga Invocation and why we chant it.  It's a good question because the translation doesn't make a lot of sense if you don't know the meaning behind the metaphors.  So, with the help of Tim Miller's translation I'll give you my take on these powerful phrases.

Translation by Tim Miller
I respectfully bow to the lotus feet of my teacher, who teaches the knowledge of the Self that awakens us to great happiness, who is the Jungle Physician and dispeller of the poison of conditioned existence.  Taking the form of a man up to the hands, holding a conch, a discus, and a sword and having a thousand heads of white light, Patanjali, I bow to you.
My translation of Tim Miller’s translation
In the first four lines we are acknowledging with humility and gratitude the lineage of the teachers who have passed down this gift of yoga.  The Jungle Physician is a metaphor for the teacher.  Conditioned existence is your life experience.  Life experience is all of the places you have been and things you’ve seen.  The life experience becomes conditioned when a judgment becomes attached to it… good  (“I like/desire this”), bad (“I dislike/have aversion to this”), or neutral.  The poison of conditioned life experience is being caught in the cycle of desire and aversion, which creates suffering.  It is a habituated pattern of seeing.  The teacher helps give us the tools for us see the truth beyond this.
The second four lines are recognizing the human and divine form of Patanjali, the author of the yoga sutras.  His hands represent his gifts to us: a conch, which represents sound to wake us up, a discus representing the cyclic patterns of nature or the acceptance of change, and a sword, which represents discernment, the ability to judge well, to cut through the patterns of conditioned existence.
It is in my experience that the truest teacher is the practice – your practice.  Amazing people who are doing their best to share that light with you have held the torch.  Teachers inspire us to practice and act to guide us to help us find our heart and know our truth, to find our greatest teacher; the teacher within.  

So, why do we begin our practice with this chant?  

Well, that answer is truly up to you.  I encourage you to think about what it means to you and how chanting it sets the sacred space for your practice.  For me it is about humility, connection, respect and gratitude.  Beginning my practice with this chant reminds me why I'm practicing; to end suffering.  Funny, after over 20 years of practice I still have to remind myself of that.  It also reminds me that I'm not alone on my journey; the practice and my teacher are here to help.  The longer I practice, the more I respect the teachings, all the teachers, and my personal teacher.  Without our teachers, this system would fly away like dust.  And, the precious gift of yoga would be lost.  The final OM seals the deal for me, it creates the magic of the practice; reminding me that all beings are connected.  

May your practice feed our soul!
Shelley Enlow