A Saint is a (usually dead) Christian venerated and considered specially holy by other Christians.
Roman Catholics Pray to saints and believe the saints will intercede with God for them. Mother Teresa was declared a saint though the way she treated vulnerable people was in many ways unsaintly. Why did this happen??? The most reasonable explanation is that there is no Omniscient God guiding the Pope or the Roman Catholic Church. Roman Catholics are subject to human error over who is considered saintly as in other respects.
Christians of some other Sects and denominations believe praying to a dead saint is Idolatry. The different Christians will never agree about everything.