Phonetics is the study of human speech. Phonetics includes the study of how sounds are physically produced (by positioning the mouth, lips and tongue), and how sounds are perceived by a listener. Phonetics can be compared to phonology, which is the study of the particular sound units (phonemes) of language.

Example
The different mouth movements of the short /i/ and long /i:/ can be compared to help learners notice the difference.

In the classroom
Articulatory phonetics, how sounds are made, can be useful for teachers and learners. Problems with the pronunciation of certain sounds can be discussed in terms of how they are made with the mouth, and learners can be made aware of the differences between their first language phonemes, and English phonemes.

See also:
https://www.teachingenglish.org.uk/article/phonology

Further links:
https://www.teachingenglish.org.uk/article/pronunciation-poor-relation
https://www.teachingenglish.org.uk/article/teaching-speaking-unit-3-individual-sounds