What's the difference between WaitForChild and FindFirstChild?

FindFirstChild returns the child if it can find a child of the instance you call it on with a name equal to the string you provide.

local child = instance:FindFirstChild("ChildName")

If it can't find the child it returns nil

You can use instance.ChildName but this will error if ChildName is not a valid member of instance. To get around the error you use FindFirstChild with a if statement

local child = instance:FindFirstChild("ChildName")
if child then --ensures child is not nil
    print'child exists!'
end

WaitForChild yields until it can find a child of the instance you called it on with a name equal to the string you provide

local child = instance:WaitForChild("ChildName")
print'child now exists!'

This is used if you want to yield code until a child exists