Diacritics are symbols used in phonology and phonetics to indicate a specific quality of a sound or syllable. They may be placed higher (in superscript) than the sound/syllable, lower (in subscript) above, below or through it.
a) Syllable diacritics showing stress and intonation.
A high slash before the syllable is used to show primary stress and low slash before the syllable shows secondary stress. For example, the word yesterday has primary stress on the first syllable and secondary stress on the last : /ˈjestəˌdeɪ/
Similar diacritics can be used to show pitch change direction on the tonic syllable :
fall \ rise / rise-fall ^ fall-rise v level ̄
The symbols shown here are those often used in phonology. As always, this is a simplification in terms of the full phonetic analysis which uses different and far more diacritics.
b) Diacritics showing qualities of individual sounds.
Phonology (which focuses on the phonemes of a specific language) uses relatively few of these when describing English. The only two in frequent use are the "length mark" ː which is used after vowels - /iː/ /uː/ /ɔː/ etc and the diacritic used to show a syllabic consonant - a vertical slash under the syllable. For example, /n̩/ as in the common pronunciation of fish and chips /fɪʃ n̩ ʧɪps/ when the vowel and final consonant are elided.
However, phonetics, which is concerned with the exact qualities of speech sounds, and with sounds used in all languages, uses many more. For example, in phonemic script , the words pin and spin would be transcribed /pɪn/ and /spɪn/. However, the two /p/ sounds are actually different in quality: the /p / in pin is aspirated while the /p/ in spin is not. In phonetic script this aspiration would be indicated by a /ʰ/ diacritic : [pʰɪn] and [spɪn]. Other symbols indicate that a sound is dental - eg [ t̪ ] ; nasalised - eg [z̃]; labialised [kʷ] - and many other features. You can find the full IPA chart, including a list of diacritics (scroll down), here.