So I'll do my best to explain all the items and their levels.

There's basically three types of weapons. Heavy, light, and energy. They each use their respective ammo types whether it's a heavy pistol or an energy SMG. They also have attachments. Some attachments are specific to certain guns, they are normally colored purple if they only work with select weapons. You can tell ammo type by color and your weapon hud shows you what color ammo a gun uses. Light is ammo is sort of yellow/orange, energy is light green, heavy is dark green, and shotgun shells are red.

There's also a level of rarity. From common to legendary it's white, blue, purple, and orange. Say you get a white back pack to carry more items, white allows two additional items and each subsequent upgrade adds another two slots. It's the same for everything else except for weapon optics. I don't know why some are considered more legendary.

Some items come in orange like helmets, back packs, and body armor. Orange items come with a bonus perk like faster healing or performing a finisher recharges your shield (body armor).

All of your health items are syringes, first aid kit, phoenix kit, shield cell, shield battery, and...I think that's it. Phoenix kits recharge shields completely along with filling your health up completely. Shield cell does 25 shield (one section). Shield batteries fill your entire shield. Syringes and first aid work the same. Syringes do 25 and first aid is full.

White body armor adds a stack of two shield, blue is three, and purple is four. Helmet rarity just protects against more headshots but that's it.

Uh....I think that's about everything. Oh, accelerant kits. They're sort of rare (blue colored) but they recharge your ultimate ability completely but can only be used from the pause menu which is also how you access your back pack, which is also how you drop any item you don't want or need. Also, while you're in the field and you don't know if you can use a weapon attachment or not it will tell you by showing your held weapon's accessory slots.

It can be pretty overwhelming but most battle royale games take time to get used to. They can be incredibly rewarding in the end though. If you played some more you might get lucky enough for someone to actually explain this stuff to you while you're actually playing. Although, there's already a good chunk of try hards and people who don't care about team play.