This basic set of features in the first layer of a CNN would be feature identifiers such as: small curves, tiny lines, color splotch, small distinguishing features (in the case of image classifiers). The filters will convolve around the image looking for similarities. The convolution algorithm will take the filter and multiply-sum the resultant matrix values. Filters activate when a specific feature results in a high-activation value:
First Layer of CNN. Here, large primitives are used to pattern match input.