Sutra:
188. kIzhE sila upAyavisEshangaLai upadhEsikka, avai dhuccagangaLenRum SvarUpa virOdhigaLenRum ninaiththu sOgAvishtanAna arjunanaik kuRiththu avanudaiya sOga nivruththyarththamAga ini idhukkavvarugillai ennalAm- padiyAna caramOpAyaththai aruLis seygaiyAlE carama slOkam enRu idhukkup pErAyirukkiRadhu.

Sri PBA Swami's Sarartha Deepikai:
Why is this called Carama Shloka? Because it shows the final means to reaching Him.

In the prior sections of Bhagavad Gita, Sri Krishna had explained other means to liberation such as Karma Yoga and Gnyana Yoga. Upon hearing them, Arjuna thought that these were not easy to follow; he also felt that for a soul, such efforts were not in tune with its nature of being completely dependent on Him and therefore were truly obstacles to the realization of that nature. Thinking that it is impossible to reach Him through such means and therefore reaching Him is a lost cause, Arjuna became grief stricken.

In order to remove this sorrow of Arjuna, Krishna taught him this final means which is easy to follow and is in tune with the soul's true nature. Since this shloka explains that final means, it became to be known as the Carama Shloka.