line, = plot(x,sin(x)) what does comma stand for?

The comma is Python syntax that denotes either a single-element tuple. E.g.,

>>> tuple([1])
(1,)

In this case, it is used for argument unpacking: plot returns a single-element list, which is unpacked into line:

>>> x, y = [1, 2]
>>> x
1
>>> y
2
>>> z, = [3]
>>> z
3

An alternative, perhaps more readable way of doing this is to use list-like syntax:

>>> [z] = [4]
>>> z
4

though the z, = is more common in Python code.