Centralisation:
this means that a country / empire is ruled from 1 place (the centre) and by one ruler (king / emperor)For example: The whole Roman empire was ruled from 1 place (Rome). Everywhere within the empire there were the same laws, rules, money, taxes. Everything was decided from Rome (the centre)
Decentralisation: this means that a country does not have one central point from where all the laws are made, but the country is divided into many different regions, each with its own ruler, laws, money, taxes, etc.
For example: Ancient Greece was a decentralised country because every city state had its own laws, rulers, money etc.