What is actually meant by the term 'stock browser' in web development?

I’m a Web Developer by profession.

Today I come across the new term ‘stock browser’ which I’ve never heard before in my 10 years of web development experience.

The text around the term ‘stock browser’ is as below :

A shim differs from a polyfill, which implements a new API that is not
supported by the stock browser as shipped.

From the above statement I didn’t get at all the meaning and context of using the term ‘stock browser’.

Can someone experienced from Web Developer community explain the meaning of the term ‘stock browser’ with proper context to me?

Thank You.

Note : If you want I can provide you the link from where I got the quoted text.