Dysphoria (also called Gender Dysphoria) is an experience of discomfort or disconnect with one's assigned gender, often accompanied by a strong desire to change one's sex to better match their identity or to be called the correct gendered language. It is often considered a medical condition. Whether or not dysphoria is required to be transgender is a hotly contested issue.
Dysphoria can be focused upon bodily attributes, treatment from others, or societal pressure to conform to expected gender roles based on their assigned gender. It is typically relieved if the person is able to transition, and therefore change their body and social situation to one which agrees with their internal identity. It is possible for nonbinary people to feel dysphoria too, for much the same reasons as binary transgender people. Cisgender may also inexperienced dysphoria for example males having man boobs and females having facial hair.
Dysphoria is actually the opposite to 'Euphoria', or pleasure. It is thus the absence of pleasure, and is not directly attached to gender identity.
Top dysphoria is a term most often used to describe someone's discomfort with their chest and upper body, such as transmasculine people feeling top dysphoria because of their breasts being too big/their shoulders being too slender, or transfeminine people feeling top dysphoria at their lack of breasts/masculine shoulders and arms.
Bottom dysphoria is a term often used to describe one's discomfort with their genitalia mismatching their gender identity.