Please do a little experiment. Hold your hand a thumbs width from the front of your mouth and say the words "kin" and "skin". You probably think that the "k" in both words are exactly the same.

If you are like most English speakers you will feel a stream of air when you pronounce the "k" in "kin", but not when you pronounce the "k" in "skin".This is because the first one is what linguists call aspirated while the second one is unaspirated. In the first case the diaphragm pushes out air while you say the consonant. In the second case it moves very little air. Still, you experience them as the same, because the sounds are allophones in English.

In some other languages, aspirated and unaspirated consonants are not considered to be the same and swapping one for the other may change the meaning of a word. For other words it may just sound wrong. For instance, the "p" in Paris is pronounced aspirated in English, but it is always unaspirated in French. Few English speakers notice this, and their accent when speaking French gives them away immediately.

Wikipedia has more information on Allophones