An asterisk is a symbol a little like a star that is used to mark a footnote, draw attention to something, or mark when something might be ambiguous.
You do something briskly when you do it quickly, sharply, and with purpose. The woman in the picture is taking a brisk walk. The opposite of brisk is leisurely.
A disk is a three dimensional shape that is round and mostly flat. Things that are disk-shaped include plates and frisbees.
An obelisk is a 3 dimensional shape with four sides, tapering towards the top. Obelisks are usually tipped with a pyramid. They are a popular shape for monuments.
Risk is a measure of the chance of something going wrong, and what bad things will happen if it goes wrong. For example, you have a higher risk of having a heart attack as you get older and fatter. Risk is also exposing yourself to the possibility of something bad happening, for example the man in the picture is risking falling off the ladder by standing too high on it to hang his wares.