Unvoiced consonants are consonant sounds that are made without vibrating the vocal chords. They can be compared with voiced consonants.

Example
Unvoiced consonants include:
/p/ as in 'pet' /t/ as in 'top' /k/ as in 'cat'

In the classroom
One way to show learners the difference between voiced and unvoiced consonants is asking them to say contrasting sounds in minimal pair words, for example 'sip' and 'zip'.

Further links:
https://www.teachingenglish.org.uk/article/voiced-unvoiced-consonants
https://www.teachingenglish.org.uk/article/phonemic-chart
https://www.teachingenglish.org.uk/article/integrating-pronunciation-classroom-activities