Typically, when using the word apprehension, you are referring to being in a state of trepidation about whether to do something or not. For example, someone could be apprehensive about flying in an airplane for the first time, and they may feel apprehensive about it.
However, it could also refer to a mental state in which we are conscious of something yet fail to fully understand the concept of what is being said or done and not passing any judgment. Comprehension also means understanding something, particularly in the fields of education or psychology.
It involves a psychological process in which a person is aware of an abstract or physical item, such as a message. It also includes how a person might perceive a situation, and if he or she recognizes a command or instruction.