oxidation and reduction

oxidation means in a reaction, an atom has increase in oxidation no.
reduction means in a reaction, an atom has decrease in oxidation no.
They must happen at the same time. Like
4K+O(2)---->2K(2)O
K has oxidation as it's oxidation no. increase from 0 to +1
O(2) has reduction as it's oxidation no. falls from 0 to -2
oxidizing agent is O(2) in the above example as it oxidizes K while reducing agent is K
The atom has decrease in oxidation no. is oxidizing agent.
The atom has increase in oxidation no. is reducing agent as it reduces another atom.

Acidify means treated by acid. Like acidified KMnO(4)
The reaction of it is accompanied with H+ ion