Learners get together with open minds to produce as many ideas as they can to address a problem statement in a judgment-free way:
there are no limits
no idea is a bad idea
come up with as many ideas as possible
the ideas can be products, systems, experiences or services.
"It's not about coming up with the right idea, it's about generating the broadest range of possibilities."
- d.school, An Introduction to Design Thinking PROCESS GUIDE
There are a lot of techniques which can support idea generation. Explore different ones to see what works best for your learners.