what does it mean when one variable has multiple variable read description?

In Lua you can return multiple values in a function (this is quite rare in programming languages) and other languages get around this by returning an array/list of values.

In the case of FindPartOnRay if a part intersect the ray then additional information is included that developers may or may not use.

I think you might be confusing a function which can return different types with a function that can return multiple values.

Example

-- multiple return <string, string>
local function a() return "A", "B" end

print(a())

-- different return types <string, boolean>
local function b(val) 
	if val then
		return "A"
		
	else
		return false
	end
end

print(b(true))
print(b(false))

Hope this helps.