The basic syntax includes a control variable , a start value, an end value, and an optional increment value.
Code Sample -
for i = 10 , 1 , -1 do
-- code
end
If the incrementation value is not defined it will be +1 by default.
I assume you know but yet , Roblox provides the documentation about almost every Development related matter from Luau syntax to all about Instances. Thus that is the best place to learn from or when you are confused.
Link: Lua Loops