One of the most important themes of existentialism is the importance of the individual, and existence as the predecessor for essence or humanity. This focus on the individual leads to additional themes regarding the importance of freedom and choice for that person, which results in feelings of anxiety and dread based on such personal responsibility. Meaninglessness is important, since these other themes of existentialism indicate that there is no inherent meaning to anything, and that it must be created by the individual. This is often associated with absurdity because of humanity's place within a meaningless universe.
The various themes of existentialism can be difficult to deal with together, so looking at each one individually can simplify the process. Individuality and existence are among the most important themes and typically help make the others easier to understand. Existence is often focused on more than the idea of the "essence" of a thing. A person as an individual is important simply because he or she exists, and his or her "essence" develops over time through life, rather than as a preexisting condition inherent to his or her existence.
From these two themes of existentialism, freedom and choice become defining elements of this existence. Since there is no inherent essence to things, there is not an inherent morality to the universe, and so each person and society must establish and assert its own moral system. This means that people have a great deal of freedom and choice, perhaps even absolute freedom, and must make choices to determine their own paths and meaning.
The freedom a person has can create anxiety and dread as he or she becomes aware of it. These feelings are important themes of existentialism since a great deal of thought has been put into dealing with them. Much personal responsibility is required for someone to come to terms with absolute freedom, and this can be difficult for many people.
Meaninglessness is also important in dealing with themes of existentialism, and it connects to the focus on existence over essence. Just as a rock has no inherent morality or meaning, everything else in the universe can be seen in a similar way. Meaning is then created by each individual. The rock might be part of a wall or used to sharpen a knife; bonds between friends and family members develop in similar ways, through choice and purpose rather than intrinsic value.
All of these various themes of existentialism are typically associated with absurdity. This stems from attempts to assert purpose and reason upon a universe that is meaningless. Conflict between people's assumed role and importance and the actual lack of meaning is the foundation of absurdity. Rather than all of this being seen as a purely negative concept, however, it is important to acknowledge that people move through and beyond this absurdity and do create positive meaning for themselves.