Randomly giving item with percentage?

If you have a probability decided of something being chosen from the rare set, you can have

local Common={"CommonAxe",CommonAxe=blah.blah}
local Rare={"RareAxe",RareAxe=blah.blah}
local rarity=1/5 -- 1 in 5

local itemset=math.random()<rarity and Rare or Common
local item=itemset[itemset[math.random(#itemset)]]

where the rare set is chosen by chance and something random is chosen out of the chosen set.

If you want each item to have its own chance/weight of being chosen, you can do this:

local items={
	{w=4,item=CommonAxe},
	{w=1,item=RareAxe}
}
local wsum=0
for i=1,#items do wsum=wsum+items[i].w end

function chooseItem()
	local itemn=math.random(wsum)
	local n=0
	for i=1,#items do
		n=n+items[i].w
		if n>=itemn then
			return items[i].item
		end
	end
end

A random number is chosen from the sum of weights of items, and the weights are added in order until the sum reaches the random number chosen. The weight landed on corresponds to the returned item.