What kind of data cannot be compressed by using the huffman codes and why? I tried looking for the answer, but I only came across loss-less and lossy compression. I don't really understand what types of data cannot be compressed by huffman and why they can't be compressed using it. 
What data cannot be compressed by huffman codes?
compression
Huffman codes have their basis in the probability that a given character will appear in a sequence. This is why when generating a Huffman prefix tree, the most common characters (those with the highest probability of appearing) are given priority for the shortest prefix.For example, in the sample text ABAABACABEDCA the character 'A' appears 6 times and will be given the shortest Huffman code, 0. 'B' appears 3 times, and will get the next-shortest code, 10, and so on, down to E and D, which both appear only once and will get the longest codes.In a truly random data set (that is, a string where all possible characters have an equal probability of appearing), then no Huffman tree that can be generated from that string will be any more efficient than any other.The reason for this is the entropy of the string. A string with high entropy can't be compressed, because more data is required to describe the entropy than would be needed for low-entropy, or highly-structured, data.The Youtube channel Computerphile has an excellent video that describes this problem quite elegantly.