to be sanctified and set apart -What does the Bible mean by that?

To be sanctified is to be “set apart.” Synonyms for sanctified are holy, consecrated, and hallowed. The Bible speaks of things being “sanctified,” such as Mt. Sinai (Exodus 19:23) and gifts to the temple (Matthew 23:17); days, such as the Sabbath (Exodus 20:8); names, such as God’s (Matthew 6:9); and people, such as the Israelites (Leviticus 20:7–8) and Christians (Ephesians 5:26).