检查元素是否存在于关联数组中

我有一个像这样的关联数组

Array=(["one"]=1 ["two"]=2)

经过一番谷歌搜索是否在数组键中找到“一个”，我发现

if [ ${Array["one"]+_} ]; then echo "Found"; else echo "Not Found"; fi

It works as I want but I don't understand the +_ right after Array["one"] What its meaning?