ye tvaksharamanirdeshyamavyaktam paryupaasate |
satvatragamachintyam cha kootashtamachalam dhruvam || 3 ||

And, those who steadfastly worship the imperishable, indefinable, all pervading, inconceivable, unchangeable, immovable and eternal.
ye : those who
tu : and
aksharam : imperishable
anirdeshyam : indefinable
avyaktam : unmanifest
paryupaasate : steadfastly worship
satvatragam : all pervading
achintyam : inconceivable
cha : and
kootashtam : unchangeable
achalam : immovable
dhruvam : eternal
In response to Arjuna’s question, Shri Krishna earlier described the seeker who worshipped Ishvara as “saguna”, an entity with form. In this and the next shloka, he described the seeker who worships the “nirguna”, the formless Ishvara. Shri Krishna wants to clearly differentiate the formless from the formful, so he provides a list of adjectives to describe the formless Ishvara, to the extent that it is possible to do so.
“Aksharam” refers to that which does not decay, that which is imperishable. The seeker negates everything that he encounters as perishable, so only the subject remains. “Anirdeshyam” is that which cannot be described or defined by the speech and mind. “Avyaktam” refers to anything that is not accessible to our senses, something that is invisible. “Sarvatragam” is that which is not limited by space, that which pervades everywhere and everything. “Achintyam” is that which cannot be conceived as a thought by the mind.
Anything that is filled with fault is called “koota”. So that by which the defect-ridden maaya and its activities look real is called “kootastha”, the foundation or base on which maaya appears. “Kootastha” also means anvil which denotes changelessness in time. “Achalam” refers to fixity, changelessness in space. “Dhruvam” is that which is eternal and deathless. In this manner, a seeker worships the formless Ishvara.
Let us also understand what is meant by “upasaanaa” or meditative worship, since that is the theme of this chapter. Shankaracharya gives a long definition in his commentary. Upaasanaa literally means to sit near. Here it refers to the seeker taking on the quality of the object of worship by moving his mind as near to that object as he can. The object of worship should be selected with the advice of the guru and scriptures. It should not be arbitrary. Then the seeker should continuously think about the object, just like an unbroken stream of oil poured from a height.
Shri Krishna continues the description of nirguna upaasakaas, seekers of the formless Ishvara, in the next shloka.