Asteya (अस्तेय): non-stealing. This one is pretty self-explanatory. However, it is good to bear in mind that there are many subtle ways to appropriate what does not belong to us. As for the other yamas, much self-analysis will be necessary to catch the subtle lower tendencies of our mind.
Desire or want is the root cause for stealing. Swami Sivananda
Asteya also includes the concept that you should try to be content with what comes to you by honest means. If you are always dwelling on things other people have and you don't have, eventually the thought of taking something that doesn't belong to you becomes more acceptable to the mind, which can in turn lead to actual theft. When you eventually convince yourself that someone else "has so much and won't miss this if I take it", you are giving yourself permission to steal. This method can reduce feelings of guilt in the short run, but in the long run is still a violation of this yama.