An allophone is a variant of a phoneme. A phoneme (a speech sound which can create meaning) may have several variants, which 1) may be used in different linguistic contexts (see the first example below), and/or 2) which may be used in the same linguistic context but in varieties of the language (see the second example).
1) The phoneme /p/ may or may not be aspirated : eg in "pin" it is aspirated - [pʰɪn] - while in "spin" it is not [spɪn]. But substituting one for the other never changes the meaning. If you say [spʰɪn] it may sound a bit odd, but it doesn't create a different word.
2) Some allophones may be used in specific varieties of English. Eg two allophones the phoneme /t/ are the glottal stop [ʔ] which is common in eg London English, and the alveolar flap [ɾ], used in many N. American varieties of English. A speaker using these allophones will be clearly recognisable as from a specific region - but the meaning of the word will not change because of the use of the allophone.
Notice that allophones are described phonetically and square brackets are used, while the phoneme, which is an abstraction is described phonologically and slashed brackets are used to indicate it. So - the allophones of /p/ may be [p] or [pʰ], while the allophones of /t/ include [t], [tʰ], [ʔ] and [ɾ].
- Native speakers are generally completely unaware of allophonic variation, or if they perceive it (more likely in the case of regional variations) may mis-perceive the sounds - eg believing that N. American speakers are producing /d/ rather than [ɾ]
- A difference in sound quality which is a matter of allophonic variation in one language, may result in a change of meaning in another - ie in that language the two sounds will be different phonemes. For example, the difference between aspirated and non-aspirated /p/, which is mentioned above, is allophonic in English, but causes a meaning change in eg Korean, Thai and Icelandic. The two sounds are therefore different phonemes in those languages.
- Language learners often have problems with sounds which are phonemes in the target language but "only" allophones (and possibly therefore not perceived) in their own language. For example, Japanese or Korean learners of English will have difficulty pronouncing /l/ and /r/ which are phonemes in English, but allophones in Japanese.
For a fuller explanation of allophonic variation, see here: