They tend to be short, dynamic activities. Warmers can be compared to coolers, which are short activities to finish the class.
Example
The learners are going to read a text about computers in the lesson. The teacher asks them to change their seating and sit down in an order based on how much they use a computer. This requires them to move around and talk to each other.
In the classroom
Warmers and coolers are not isolated activities but can serve to introduce a topic or to recycle some previously learnt language in an enjoyable way.
Further links:
https://www.teachingenglish.org.uk/article/warmers
https://www.teachingenglish.org.uk/article/change-places%E2%80%A6