What is a "Fellowship Offering"?

What is a “fellowship offering” (Leviticus 3:1)?

In the phrase “fellowship offering,” the word translated “fellowship” includes the ideas of health, wholeness, welfare, and peace. It is reflected in the common Jewish greeting “Shalom!” This offering apparently symbolizes peace with God because the worshiper joins in the sacred meal (symbolized sharing a meal with the Lord). A fellowship offering could be voluntary as a special offering of thanks to God or could be given as the result of a vow or as a freewill offering (7:12-26). This offering was given by the thousands at special celebrations when many people joined in the sacred meal (1Kings 8:63). If a man was too poor to bring a voluntary fellowship offering, he would probably be given a share in the offerings of others

– NIV Bible Commentary