All actions have a consquence- karma
The theme or belief of paying for one's actions or sins is reflected in the Upanishads through the concept of 'Karma', which means work. This means that an individuals every intent and action will endure consquences, be it good or bad. Moreover, he who lives life doing the right deeds for the betterment of everyone else will, in his next life, be born in favourable cicumstances. He who enganges in destructive behavior, or sinful acts will later be born under unfavourable circumstances in his next life.
Reincarnation- samsara
The theme or belief of reincarnation is clearly reflected in the Upanishads through the concept of 'Samsara', which means wandering or world. It is the concept that every living being will, after death, come back to life on this Earth in some form, be it animal or human. The cycle of death and reincarnation is never-ending. The circumstances the individual will be born into, in his next life, depends upon his acts from his previous life.
Duty- dharma
The theme of duty or correct behavior is reflected through the concept of 'Dharma', which means 'way of righteousness' in Sanskrit, the language that the Upanishads have been written in. This means that each person has a set of responsibilities in accordance with his or her caste, which is the four-fold division of the Hindu soceity, namely, Brahmind, Kshatriya, Vaishya, Shudra. All the castes had certain occupations particular to them. Brahmins were the givers of knowlege, Kshatriyas are the protectors, Vaishyas are the merchants and Shudras are the labourers. Each of these had to perform their assigned duties which must be fulfilled.
Liberation- moksha
The theme of liberation is also reflected in the Upanishads through the concept of 'Moksha', which means liberation or release. It is the release from the cycle of rebirth. It is the release and freedom from the never-ending cycle of death and reicarnation. It is the ultimate aim of an individuals mind and soul, according to the Upanishads. It is the end of rebirth, it is when the individual breaks away from the cycle of death and rebirth and finds solace.