A warmer is an activity at the start of the class to warm up the learners. They tend to be short, dynamic activities. Warmers can be compared to coolers, which are short activities to finish the class.
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.