From Teflpedia

Emphasis (/ˈemfəsɪs/; plural when countable emphases (/ˈemfəsi:z/)) is special stress given to syllables, single words or phrases over what would normally be expected.

In speech, this can be done by making the word louder. The stressed word can be ifferent to the one normally stressed.

e.g. in the phrase "she didn't call me", "call" is usually stressed. However, "she" can be stressed (implying she didn't call me but someone else did), or "me" can be stressed (implying she called someone else); or both "she" and "me" can be stressed (implying she didn't call me, but that I called her).

In writing, this can be done by use of exclamation mark(s), underlining or emboldening or italicisation, or ALL CAPS, OR A COMBINATION OF THESE!!!