Kerning is the process of adjusting the space between letters in a font. This is most frequently used to change the aesthetic of a typography or when creating a brand new font style. Effective kerning can create interesting fonts to read but ineffective kerning can be quite cumbersome to read and detract from the design as a whole.
Emphasis can mean any number of things. As it applies to design, emphasis can be a focal point (i.e. where the eye is drawn to in the image) or it can be changing fonts to appear larger or bolder than others on the page. A graphic designer will use emphasis to lead the viewer around his/her design in the way intended to garner the most interest in the product.
Descender is a term of typography. It refers to the portion of lowercase letters that extend beyond the page line (referred to as baseline in some media). Some examples of this would be the lowercase “q” or “j.” Various fonts can play with the descenders and make them longer, shorter, or more elaborate. Cursive fonts tend to feature the most prominently emphasized (call back to the previous term!) descenders of most fonts.
Visual Impact refers to the effect media or design has on our senses. Digital media as it pertains to the visual arts makes most use of our sense of sight almost exclusively, though hearing is utilized in music and film etc. A designer has to convey all of his ideas and emotions through the use of sight. When you look at a design, the emotions you feel, the things you imagine, the memories you recall, all of these are the visual impact of the design.
Constraint(s) serve to limit the designers actions and offer guidelines for how to go about creating their works. This can be any number of these from directives from a superior, page sizing, software required, client requests, and so on and so forth. Constraints tend to carry a negative connotation with them but they serve a useful and positive purpose of guiding or directing a designer towards the correct path for his/her design project.