self.strength means there is a local variable on that object used there.
for example if you have a sprite named char you don't have to create global variables to keep track of his life damage or strength
you can use a local variable on the sprite itself and give it a value, then u can use those values by using an action or condition like
sprite.hero is self life =< 100
and
sprite.hero on collision with refill health item
then do action sprite.hero set self life to self.life+healthitem.liferefil value
that is the formula of image points used in this case to set the position of an object to a pivot point
formula for image points
thanks to 
newt on this one he recently posted in general
xhand is a global variable with the X coordinate of the characters hand in this case or a point position near the hand of the character.
is cosinus from
where
should be equal to
(i don't know where you get the angle value on this one though in your current case)
yhand is the global variable with y coordinate of the characters hand as with x hand
sinus from
where
again is the angle set and used in the x positioning also
arrow.str -local variable str on arrow object, is the strength or in the image point formula that is a distance value.
distPivot global variable that holds the distance of from or to a pivot point, this should be set somewhere in the code or already has it set by default in the global variables list.
this sets the position of the arrow when the bow is shooting basically. going back and forth nice little trick to animate a movement of an arrow before it flies off from the bow.
sets the power /speed at which the arrow will travel at probably, by comparing if strengths value which is probably set while keeping pressed down a button or the touch on screen. it also influences the X position on the xhand-cos(a)*arrow.str/10