[OJFord]

GP said indentation markers - typically vertical lines rendered on the background on every line at every indentation level before the first character of code.

Like this, (but the | is not in the source):

    def fun()
    |    return 'blah'

    class Foo:
    |    def __init__(self):
    |    |    return None

Often it'll be a more subtle colour than the code so it's less visually heavy than presented here. In vim I have it only shown in normal mode, to avoid confusion while editing.