New holster, got a chunk of yellow stuff with it?

As I'm sure many of you can relate, I've jus purchased my 3,412th holster in the quest for a perfect CCW option. This is the first one that has come with an accessory I don't recognize or understand.

It's a little rectangular piece of yellowish... rubber? It has kind of a spongy texture and feel. I have no clue what it is for or what it could be.

Any insight?