FOAM
Different densities of foam are used for different types of mats.
Gymnastics Mats: mats gymnasts use to train with. For example, when a gymnast is on the bars and preping for a dismount she can't do perfectly yet, there will be a big mat underneath for her to land...
Normally a low density Polyurethane (foam) covered in vinyl.
If you are old enough you can buy them online. Or you can ask your local gym.
a hard ball thing then that is covered in string
a lot of people mistake it for led but it is in fact graphite
Benefits of Anti-Erosing Matting
How to Make a Chest of Drawers from Recyclable Materials (with subtitles)
How to Crochet a Cat or Dog Mat
How to Recycle Old Chopsticks Into Ornamental Coasters