What is a 2 handed weapon and 1 handed weapon beceause tag "mainhand, offhand" is confusing me 

might be a bug here : I have a staff that said "mainhand, offhand" and it's red with empty offhand and when I equip an offhand it say "missing requirement"

The red background on items means that you do not have enough stats (str, dex, or int) to make that item be active. You have a Main hand slot and an offhand slot. An item that has mainhand,offhand can be placed in their of those two slots.