Name of and reason for Python function parameters of type `name=value`

It's entirely possible that this question is a duplicate, but I don't know what this concept is called so I don't even know how to search for it.

I'm new to Python and trying to understand this function from a Caffe example:

def conv_relu(bottom, ks, nout, stride=1, pad=0, group=1):
    conv = L.Convolution(bottom, kernel_size=ks, stride=stride,
                                num_output=nout, pad=pad, group=group)
    return conv, L.ReLU(conv, in_place=True)

I figured the parameters stride=1, pad=1, etc in the conv_relu function definition are default initial values, but then what do kernel_size=ks, stride=stride, etc in the L.Convolution call mean? Is it kind of like a name/value pair?

If nothing else, can someone please tell me what this is called?