A sound change in a language is when the sounds of the language become different overtime. Since people of different languages or dialects often talk to each other, the way people talk becomes more like the way the others talk. Because of this, it is natural for languages to sound differently overtime. Sometimes it happens slowly, while in other times it happens quickly.
An example of a sound change in English is the Great Vowel Shift, when all of the long vowel sounds in Middle English changed into what they are today. This is why the way English is spelled is so differently from how it is spoken.
Spelling systems that do not change with the sound changes of a language are often more difficult for a learner to learn how to read in that language. These kinds of spellings are called fossilized spellings. These include spelling systems like English, French, Mongolian script, and Thai. All of these spelling systems have changed little for the past few hundred years, even though their spoken languages sound very different from how they used to sound. Spelling systems that change with the sound changes are often easier for a learner to learn how to read. These languages include Japanese, Turkish, and German.
Another much smaller sound shift in many dialects of English is the cot-caught merger, where words with lower back vowels like in cot and caught sound exactly the same to native speakers of certain English dialects and are spoken without any difference between the two sounds.