Integration is the "opposite" of differentiation.
If the derivative represents the rate of change of a function, the integral represents how much that function has changed across some interval.
No idea. There's some things behind this but to be honest not one time has the riemann sum been asked in an exam, so we're skipping this. All you have to know is that integration via riemann sums involves breaking up the graph of the function into tiny rectangles to calculate their surface, then adding them all up to approximate the value of the integral.