This is the first of the Yoga Sutras (which is the yoga text written by Patanjali). Broken down in plain English, it means now begins the study of yoga. At first glance, the statement simply states here is the information on yoga. Like many yogis, I tend to look into this statement a little deeper. So much so, that I got the sanskrit phrase as a tattoo on my arm.
Atha, or now. To me, this means constant practice, not just when you're on the mat. Think of the cell phone commercials, but instead of "can you hear me now", think "are you practicing now". Try to stay in a consistent mindset. One way of putting this into practice is by not judging yourself (or anyone else) when things don't go as planned. The practice is now - in the present.
Yoga means union. So, yoga is a lot more than just bending your body in strange shapes. That union may be that of your mind and body, your breath and spirit, your connection with other beings or a higher power, or all of the above. The interpretation can be up to you on this one.
What do I mean with all of this? Simply take your yoga practice and make it a part of your whole life. Live the yoga, regardless of what is happening in your physical practice at the moment. Be a yogi 24 hours a day, not just during your time on the mat.
Namaste - Beth