From Blaseball Wiki

Items were introduced at the end of Season 15 as a result of the Bats Decree, which granted all active (non-Shadows) players in the ILB a Bat item.

Players can hold one item for every evolution level they have - so while most players can only hold 1 item, Evolution 1 players can hold 2 items, and so on.

Items can have an effect on the hitting, pitching, baserunning or defense ratings of the player holding them, and also have unique properties:

Durability If this value reaches 0, the Item is broken
Type Bat, Cap, Sunglasses, Ring, etc
Elements Extra properties of this Item like "Cold", "Inflatable", or "Plant-Based"

Generic Items have a simple name like "Bat" and affect a single attribute.

Type Attribute
Bat +Thwackability
Board +Groundfriction
Cap +Unthwackability
Field +Tenaciousness
Glove +Omniscience
Jersey +Musclitude
Necklace +Moxie
Ring +Ruthlessness
Shoes +Laserlikeness
Sunglasses +Cinnamon

Rare Items also exist, which possess extra Elements that affect more attributes. Each Element also adds 1 additional durability. Items can even have more than one Element at a time, such as the Sharp Gravity Holey Glove, which has a Durability of 4 and the Sharp, Gravity, and Holey Elements.

Element Modification
Ambitious +  Ambitious
Blaserunning +  Blaserunning
Careful +  Careful
Chunky +  Chunky
Clutch +  High Pressure
Entanglement +  Entangled
Force +  Force
Golden +  Super Idol
Gravity +  Gravity
Noise-Cancelling +  Soundproof
Observation +  Observed
Parasitic +  Parasite
Repeating +  Repeating
Skate +  Pro Skater
Traveling +  Traveling
Uncertain +  Uncertain
Element Batting Pitching Baserunning Defense Miscellaneous
Arm +Thwackability +Unthwackability
Blood +Divinity
Brambly +Ruthlessness +Anticapitalism
Chaotic +Moxie
Cool +Moxie +Coldness +Laserlikeness
Cryogenic +Coldness
Dexterity +Thwackability +Continuation +Omniscience
Frosty +Coldness +Chasiness
Greedy +Basethirst
Hard +Musclitude +Ruthlessness -Omniscience
Head +Moxie +Omniscience
Hearty +Indulgence +Tenaciousness +Pressurization
Holey +Divinity
Hot -Moxie
Inflatable +Buoyancy
Intelligence +Ruthlessness
Leg +Laserlikeness +Chasiness
Limestone -Overpowerment
Lucky +Divinity
Mesh +Moxie -Overpowerment
Metaphorical +Shakespearianism -Continuation +Chasiness
Passionate +Basethirst
Plant-Based -Buoyancy +Suppression +Watchfulness
Rubber -Overpowerment
Sharp +Thwackability +Unthwackability +Groundfriction -Tenaciousness
Strength +Musclitude +Overpowerment
Vanity -Martyrdom
Weird +Patheticism
+Shakespearianism +Groundfriction +Cinnamon
Wisdom +Indulgence +Omniscience
Wooden +Thwackability -Laserlikeness -Tenaciousness
the Feast +Thwackability -Unthwackability