Latex tubing or Latex free tubing.
Installs onto the bow over the thumb leather to pad and add grip to the contact area of the cellists thumb.
Clear is a latex free material,  Black is Latex Latex comes in regular and extra thick.
Can be cut to length or standard is 1 1/2" as seen in photos.
To view the different types the photo with the three bows:

1. Top bow has the clear latex free tube (for those with latex allergies)
2. Center bow has the standard Black Latex tube
3. Bottom bow has the extra thick rubber.
   

NOTE: This material is not squishy or spongy.
#2 latex is the easiest to install and remove. The other two go on easy with lubrication, but are a bit tougher to remove.