Bhagavad Gita Verse 50, Chapter 18

siddhim praapto yathaa brahma tathaapnoti nibodha me |
samaasenaiva kaunteya nishthaa jnyaanasya yaa paraa || 50 ||
 
How one who has attained perfection also undoubtedly achieves the eternal essence, learn from me in brief, O Kaunteya, that supreme devotion to knowledge.
 
siddhi : perfection
praaptaha : one attained
yathaa : how
brahma : eternal essence
tathaa : also
aapnoti : achieves
nibodha : learn
me : from me
samaasena : in brief
eva : undoubtedly
kaunteya : O Kaunteya
nishthaa : devotion
jnyaanasya : of knowledge
yaha : which
paraa : supreme
 
Shri Krishna now starts to recap the content from chapters five and six. Chapter three and four covered the topic of karma yoga, and how karma yoga gradually evolves into jnyaana yoga. Once the seeker has reduced his stock of desires, has purified his mind, and has gained knowledge about the aatmaa, the self, from a competent teacher, he then enters into the stage of sanyaasa, complete renunciation of action. Chapters five and six explain the process of entering into, and maturing of, the state of renunciation.
 
Siddhi here refers to the purification of mind obtained as a result of karma yoga. Jnyaana nishthaa, devotion to knowledge, also known as nidhidhyaasana or meditation, is the culmination of sanyaasa. If we ever wonder how monks spend most of their time, this is it. Besides doing the bare minimum needed to maintain the body, the monk is engaged in one and only one thing - constant contemplation, constant abidance upon the self.
 
For most of us, just contemplating on the self may seem a bit odd. How can such a seemingly mundane engagement result in liberation? So we need to remember that the self is already attained, there is no work that is needed to attain it. The only work that we have to do is to get rid of what is the no-self, in other words, purify our mind through karma and bhakti. Shri Krishna emphasizes this point by using the word nibodha, which means to know. There is nothing else that needs to be done in sanyaasa since it is the last stage of yoga.