Something private is something you don't want to share; it's for your eyes only. Your diary entries and your bank account balance might both be kept private.
As a noun, the lowest enlisted rank is called private. You may enlist as a private but have plans to retire from the Army as a four-star general! Sometimes the adjective private is used as the opposite of public. If you own your own island, it's private property. Alcatraz Island, however, is public property and the site of a museum preserving its past life as a famous prison — former home to Al Capone.