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.
You roll when you lie down and move from your front to your back (or the other way around). If you keep rolling you can travel quite some distance. You roll a ball or something else round when you push or pull it along the ground and it moves by turning over and over without losing contact with the ground.
A roost is a place where birds go to sit, or the act of birds sitting down to roost. The chickens in the picture are roosting.
