Stress is the emphasis which a speaker gives to a certain syllable in a word or a certain word in a sentence.

The speaker says that syllable or word more loudly, slowly or carefully, or with a change in pitch (how high or low a sound is).

Word stress

The stress on a particular syllable in a word is called word stress. For example:

Word Syllables
Spelling With STRESS Total Stressed
market MAR-ket 2 1st
demand de-MAND 2 2nd
inflation in-FLA-tion 3 2nd
employee em-ploy-EE 3 3rd
economy e-CON-om-y 4 2nd
economics e-con-OM-ics 4 3rd

In some languages the stress is always on the same syllable - for example, the penultimate (last but one) syllable. This is not true of English. Instead, you have to learn where the stress is in each word.

Sentence stress

The stress on a particular word in a sentence is called sentence stress.

Content words (main verbs, adverbs, nouns and adjectives) are more likely to be stressed than grammar words (auxiliary verbs, conjunctions, prepositions, pronouns and articles). See, for example, the stressed words (in CAPITALS) in this dialogue:

Did you have a good HOLIDAY?
YES, VERY good
Was the WEATHER all right?
It was FINE for the FIRST part, but for the REST of the time it was pretty MIXED. We ENJOYED ourselves though. We had the CAR, so we were able to do some SIGHT-SEEING when it was too WET to go on the BEACH.

From: Cruttenden, A. (2014) Gimson’s pronunciation of English. 8th edn. London: Routledge. pp. 270-271.

Sometimes a word is stressed because it is particularly important. For example:

Make sure you submit your essay BEFORE 3 o’clock. And write your name at the TOP of the first page.