Acoustic features are the landmarks of word pronunciation; vowel sounds, consonant sounds, the length and emphasis of the individual sounds, and many other features. These acoustic features are the result of individual articulation (creating sounds) and can vary some between users. Words are composed of vowel and consonant sounds which are then sometimes emphasized or inflected in different ways to create regional accents, shades of meaning between words, and distinctions between similar words.
For instance, English contains many homonyms, words that sound the same but have different meanings such as 'witch' and 'which.' Oftentimes, there may be subtle differences in pronunciation (caught vs cot) or major differences in emphasis that indicate differences between the words (invalid has different meanings depending on whether you emphasize the first or second syllable).