This is probably the most quoted aphorism from one of the most iconic text on yoga 'Patanjai's Yoga Sutras.' It is also the only mention of the practice of asana, kind of ironic since asana is the main focus of modern yoga. It means: Seated posture should be steady and comfortable. In the context of Hatha Yoga (physical yoga, or the yoga of the body, not a style of yoga that many people today take it to be.) it's often understood to mean that each and every asana (posture) should be both steady, steadfast, but also contain a certain lightness, ease and spaciousness. Or another, looser translation: The connection to the earth should be steady and joyful.
Today, during my morning meditation this sutra came into my mind (as a lot of other stuff entered and exited my mind as I was trying to penetrate deeper). To me Patatanjali's famous text reads like an instruction manual on how to end suffering and the illusion of separation (ego) through the practice of meditation. So it's quite obvious that the asana he refers to is the posture one takes for meditation, so a cross-legged seat or something similar. And anyone who has tried to sit still for any length of time knows how challenging it can be for the body. The practice of Hatha yoga can help. It can help prepare the body, but it also helps prepare the mind, as a good Hatha yoga practice is a moving meditation of sorts, bringing the awareness back to the present moment. This is obviously the intended function of asana practice.
But, I feel that the shtira (steadiness) and the sukham (ease, lightness) also has to be present on a mental level when meditating. If there's no shtira I will just sit on my meditation cushion daydreaming, or I will abandon it altogether to go sit in front of the computer or go clean the toilet (which often strangely seems like a more enticing task than meditating ...). But if there is no sukham, I will be too stiff to 'go anywhere.' I need to relax, to let go a bit in order for the mind to be silenced. In order to subdue my 'vrittis.'
I also feel that shtira and sukham has to be present in life in general, and that on a more earthly level, the trick is to find a balance between the two.