Semantic features are the features that make sense to humans. If you look at activations of few last layers you will observe their visualization of activations looks like objects themselves. By looking at the activation you can tell what object it is (because this is how we perceive the world). Semantically week features are the low-level features like edge/ color blob etc. It is noise to humans, you cannot tell what object it is by looking at activations, because we humans don't perceive this way, and hence semantically weak. An example could be your mother tongue, it has got semantically strong meaning to you (you can understand and speak it well, samjhe na bhai?), "Mandarin Chinese" (it is one of the hardest languages to learn) is semantically weak to you, as if I show you few words, you won't be able to make any sense of it, to you it would be random lines making some shapes (谢谢你).