Yajna-shishta means whatever remains after you have first offered to others. The virtuous eat only after sharing. They serve the family, feed the guest, give to the community — and then partake of what is left. This practice, simple as it sounds, frees them from the weight of sin.
On the other hand, those who cook only for themselves, who consume without sharing, eat nothing but sin. The principle stretches far beyond the kitchen. Living only for yourself — keeping all your talent, time, and resources for your own benefit — is the root of what the Gita calls papa.