— श्रीमद्वाल्मीकिरामायणे अयोध्याकाण्डे द्वादशस्सर्गः (४३)
Kaikeyi reminds Dasharatha that one should always keep-up to his promise, by reciting this shloka that describes the stories of King Shibi and King Alarka.
Kshatriya is one who saves others from क्षति (destruction/damage). Hence kings like Shibi and Alarka have taken the promise that they will protect those whose ask for help.
Once Indra and Agnideva took the forms of Eagle and pigeon respectively and come to Shibi in order to test his ability to keep up his promise that he will protect the feared. On the request of the pigeon, Shibi promised to protect it. However, the Eagle raised an objection that his prey is his food and it is adharmic for the king to take away his food. Then Shibi gave away an equivalent portion of his flesh to the Eagle and protected the pigeon. Similarly, Alarka gave away his eyes to a blind Brahmin who came to him (this shows that in Ayurveda too the process of transferring eyes was known).
However painful and difficult, one should keep-up his promise like Shibi and Alarka.