A plus item gives additional points of defense or attack. A plus item, +15 is max.
From monsters and when buying items chances of getting + items are 1/8 for +1, 1/8/8 for +2 etc.
Spawning items: 1/8, 1/8/4, 1/8/4/4 etc.
Random treasure chest on overland map: 1/4, 1/4/4, etc
Treasure map chest: 1/2, 1/2/4, 1/2/4/4 etc.