What does [i,:] mean in Python?

It's specific to numpy module, used in most data science modules.

ans[i,:] = y_poly

this is assigning a vector to a slice of numpy 2D array (slice assignment). Self-contained example:

>>> import numpy
>>> a = numpy.array([[0,0,0],[1,1,1]])
>>> a[0,:] = [3,4,5]
>>> a
array([[3, 4, 5],
       [1, 1, 1]])

There is also slice assignment in base python, using only one dimension (a[:] = [1,2,3])

I guess you are also using numpy to manipulate data (as matrix) ?

If based on numpy, ans[i,:] means to pick the ith 'row' of ans with all of its 'columns'. Note,when dealing with numpy arrays, we should (almost) always use [i, j] instead of [i][j]. This might be counter-intuitive if you used Python or Java to manipulate matrix before.

I think in this case [] means the indexing operator for a class object which can be used by defining the getitem method

class A:
    def __getitem__(self, key):
        pass

key can be literally anything. In your case "[1,:]" key is a tuple containing of "1" and a slice(None, None, None). Such a key can be useful if your class represents multi-dimensional data which you want to access via [] operator. A suggested by others answers this could be a numpy array:

Here is a quick example of how such a multi-dimensional indexing could work:

class A:
    values = [[1,2,3,4], [4,5,6,7]]
    def __getitem__(self, key):
        i, j = key
        if isinstance(i, int):
            i = slice(i, i + 1)
        if isinstance(j, int):
            j = slice(j, j + 1)
        for row in self.values[i]:
            print(row[j])

>>>a = A()
>>>a[:,2:4]
[3, 4]
[6, 7]
>>>a[1,1]
[5]
>>>a[:, 2]
[3]
[6]