There is no achievement like peace (the word tapa might be used here to emphasize the efforts required to keep your mind peaceful.), there is no happiness like satisfaction, there is no disease like desire, there is no dharma like mercy.
All Upanishads are (like)cows, Gopalnandana (Shrikrishna) is their keeper. Intelligent Partha (Arjun) is the calf who enjoys the milk and splendid GeetAmRit is the milk of these cows. (Geeta is the precise summary of all Upanishadas.)
It is said that, if one dilutes milk with water, and gives to swan, swan is able to extract milk and drink it. (dudh ka dudh pani ka pani) Swan is white, crane is (also) white. (then) what is the difference between crane and swan? when it comes to extracting milk from a mixture, swan is swan and crane is crane. i.e. crane does not have this ability.
This subhashit is trying to explain that the external appearance does not make a person great but his qualities. It also explains that wise people precisely know what is useful and what is not.