what are the item tiers? as in what order do they go. plain, rare, magic, mythical, mystical, angelic.

Have a look at this list: https://ladderslasher.d2jsp.org/index.php?g=15

thanks. what does reroll on the character select screen do? respec or?

Reroll reset your character's level to 1 and you have to select which class you want to play again (Rogue, Fighter, Samurai, etc.).

You will keep all of your items and proficiencies you had on that character.

Go back to level 1 keeping all items and prof points that you have gained. Not like Respec.