They can be everyday items used as teaching aids.
Example
The learners are learning to describe clothes. In turn, they describe someone in the class for others to identify.
In the classroom
Realia can include the learners themselves, the classroom, furniture, the school, published material such as advertisements and menus, the contents of learners' pockets and bags, and the view outside the window.
Further links:
https://www.teachingenglish.org.uk/article/realia-0
https://www.teachingenglish.org.uk/article/realia-bag
https://www.teachingenglish.org.uk/article/how-create-resources-nothing
https://www.teachingenglish.org.uk/article/conveying-meaning
https://www.teachingenglish.org.uk/blogs/rachael-roberts/rachael-roberts-realia