Polysemy refers to the quality of some words to have several related meanings. A word which has several related meanings is thus a polyseme. These can be compared to homonyms, which are words that have several completely different meanings.

Example
The word 'head' is a polyseme because it has several related meanings:
'My head hurts' 'He heads the news section' 'We'll head them off at the turning' 

In the classroom
Polysemy is a very useful aspect of language as it enables learners to make informed guesses about the meaning of new words if they recognise them as polysemes.

Further links:
https://www.teachingenglish.org.uk/article/working-vocabulary
https://www.teachingenglish.org.uk/article/multi-word-verbs-learner-problems
https://www.teachingenglish.org.uk/article/do-some-words-matter-more-or-frequency-fallacy