A stress-timed language is a language where the stressed syllables are said at approximately regular intervals, and unstressed syllables shorten to fit this rhythm. Stress-timed languages can be compared with syllable-timed ones, where each syllable takes roughly the same amount of time.

Example
English and German are examples of stress-timed languages, while Spanish and Cantonese are syllable-timed.

In the classroom
Learners whose first language is syllable-timed often have problems producing the unstressed sounds in a stress-timed language like English, tending to give them equal stress.

See also:
https://www.teachingenglish.org.uk/article/syllable-timed-languages

Further links:
https://www.teachingenglish.org.uk/article/stress-timing
https://www.teachingenglish.org.uk/article/connected-speech
https://www.teachingenglish.org.uk/article/rhythm