Phonemes are said to be 'abstract'. What does the term 'abstract' mean in this context?
When we refer the adjective 'abstract' to a phoneme, we mean that the phonemes do not constitute the real sounds of which words are made. A phonemes is usually defined as the smallest linguistic unit which can make a difference meaning when joined to other phonemes. The real sounds are called 'phones'. To understand the difference, the Wikipedia entry in the first link makes you think of the different pronunciation of the K in KIT and SKILL, while the text in the second links below suggests that you think at how different the sounds produced by the letter P in the words PAN and SPAN are.