백본에서 stage의 개념이 무엇인가요?

안녕하세요 강의와 병행해서 mmdetection을 살펴보고 있는데,  백본 용어 중에서 num_stages와 out_indices 라는 것이 혹시 대략적으로 어떤 의미인지 알 수 있을까요?

backbone=dict(  # The config of backbone
        type='ResNet',  # The type of the backbone, refer to https://github.com/open-mmlab/mmdetection/blob/master/mmdet/models/backbones/resnet.py#L288 for more details.
        depth=50,  # The depth of backbone, usually it is 50 or 101 for ResNet and ResNext backbones.
        num_stages=4,  # Number of stages of the backbone.
        out_indices=(0, 1, 2, 3),  # The index of output feature maps produced in each stages
        frozen_stages=1,  # The weights in the first 1 stage are fronzen
        norm_cfg=dict(  # The config of normalization layers.
            type='BN',  # Type of norm layer, usually it is BN or GN
            requires_grad=True),  # Whether to train the gamma and beta in BN
        norm_eval=True,  # Whether to freeze the statistics in BN
        style='pytorch'),  # The style of backbone, 'pytorch' means that stride 2 layers are in 3x3 conv, 'caffe' means stride 2 layers are in 1x1 convs.