We think it means allenergy you had before the reaction is equal to all the energy you have after the reaction. It's a consequence of the observed fact that the laws of physics don't appear to change over time. It helps alot with solving problems.
How does friction affect conservation of energy?
It would only affect it by potiential energy to never go down even if the kinectic energy would go up, it wouldn't make a difference.
How can you numerically predict the velocity of a skater at the bottom of the hill?
I think that you ccan use the velocity formula. V=d/t. To find it you will need to find distance and time.
